The yeast has become sentient and is fighting for labor rights

The King has become increasingly teetotaling and has levied an ever increasing tax on beer and spirits to discourage "sin" among the people. In reality he has a god trapped under the castle and the only way to keep it contained is to keep it perpetually drunk. However its tolerance is increasing and now its dependence has reached a point where, if cutoff, its "DTs" could destroy the kingdom. The wine cellars have been totally emptied. Workers in the palace have become suspicious that the vast grates into which they empty confiscated barrels don't, in fact, lead to the river.

Beer is essential 4 things; grain, hops, water and yeast.

Grain: cartels limiting shipments out of the grain basket.

Hops: A group that is very “anti drug” destroying hops since they come from the same category of plant.

Water: an addictive adulterated is being added, so they can charge more.

Yeast: someone figured out how to combine “speak with animals” with “speak with plants” letting them speak with fungi, convinced the yeast to stop working.

A strange new product has come onto the market at the same time that someone has bought up the stocks of beer and grain.

The new brew has wonderful effect but its flavor is odd and ends with a very deep sleep. It’s cheap and widely available.

Turns out that some mind flayers or other psionic beast have been producing and distributing a concoction made from their own milk/extract.

The effects and deep sleep makes people easy and reusable prey.

Bit mundane underlying logic which can of course be embellished by the characters you populate it with: a brewer with connections to the royal court has acquired a monopoly by lobbying for brewing to only be allowed under royal warrant. They have managed to gain regulatory capture through their connections and all the other brewers are getting raided and their inventory flushed. Now the 'royal' brewer can set much higher prices without any danger of being undercut.

Or you could work it as some variant of beer purity law

Depending on how medieval the setting is, hops were not the common bittering in many beers, it was a mix of herbs called gruit. If this mix's ingredients are secret (or regulated), it would be easy to monopolize and drive the price up.

The price may be going up due to the cost of some profane adulterants being added in ever greater quantities...

a malicious magical being is siphoning magical energies from the brewing of potions, but does not distinguish between witches and alchemists making real potions, and the brewing of beer; humanoid creatures are just as strange and eldritch to this being as it is to humanoids, and the processes seem similar enough. alternatively, the brewing of beer is actually very slightly magical, and beer is brewed in such higher quantities than potions that even the very small proportion of magic it can take from that is still more net energy than it can get from actual potions. this disturbance to the brewing process is making many batches of beer taste foul, and the brewers cannot sell them; supply is low and ingredients are being wasted, driving prices up. potions are also being affected, becoming inert or harmful instead of the intended effect, but this has not been noticed by the general public because they hardly ever interact with magical potions. also because people who make potions are trying to keep a lid on the situation to avoid suspicion from the public who are already wary and distrustful of them. (not familiar with the setting you mentioned, so maybe some of these things don't quite fit, but could be a jumping-off point)

Lab engineered barrel blight is cutting off supply for brewers.

The king is charging a bubbles surtax.

Yeast is secretly being harvested from goblin tears, and the supply is running low after a goblin jailbreak.

A blight brought on by mystical means beyond the control of the hapless out of town merchant who invoked them solely for the opportunity to corner the market with his trove of inferior beer he couldn't sell elsewhere. Once the players figure out the merchant is responsible, it becomes apparent there's a far greater threat in the supernatural causes of the blight and they now must face off with them.

All the process that make beer happen originate from the God of Spirits. His partner recently broke up with him as they are going to the Far Realms and don't feel it was fair to keep it long distance. If anyone in the land wants beer again, they'll have to act like some friends and make him smile again.

Someone is stealing the grain to feed an invading army.

A brewer cabal has employed ruffians who rob taverns and smash up the kegs, driving the prices up.

The crown is trying to keep it a secret, but there's a flesh-eating disease that is spreading throughout the kingdom, and it incubates in the alehouses. Taverns and breweries are being shuttered all over with only an offhand explanation. When ingesting the infected brew, the host will be consumed painfully and noisily from the inside.

A "hero" fresh from the dungeon throws his weight around and threatens that bad things might happen if he doesn't get paid protection money. He found a magic item that makes the danger clear.

Ale Elementals (Alementals)!

There is a Drunken Master Monk so powerful that they are weakening the barriers between the material plain and an outer plain of booze.

This manifests as inebriated Alementals appearing in brewery's and distillers and causing havoc before dispersing back to their home plane.

....

Less sinister, but potentially hilarious. (Cause could be changed if you wanted something more sinister)

Here's a boring mundane reason that could be fun. Ancient Egyptian pyramid builders were given beer as payment.

You could have the local evil person have an army of builders building some evil building, supplementing their pay with beer. Such a massive work force would require a lot of beer siphoning off most of the local supply. As the beer supply dwindles, the price increases.

Government official is getting extorted by [a path to the actual plot], but they own a vineyard and have the power to levy taxes. They tax beer to drive demand for wine.

Purely selfish self-preservation.

Dwindling supply due to bbeg creating a portal to some other plane that allows demons to come through and they get out of his control and just decide to get hammered 24/7

Constructed scarcity? The brewers pour out the beer per night on behalf of insert local rich person to make the prices higher, and when that has happened they then secretly will produce more to earn A GAZILLION gold.

(Based on a conspiracy theory i heard from a wacky guy at a party)

Make the beer magically cursed so people want to pay whatever price to get more.

Affordability is essential to societal stability. Some cabal against the city government is pressuring the brewers, or the taverns, or the brewers guild, to increase beer prices. This can be made through taxes, fees, extortion... The extra money is siphoned to other actions to destabilise the city or to build a army. This can be done by kidnapping relatives to innocent people, corrupting government/guild officials, mind control...

The real motivation can be a noble dispute, a revolutionary uprising, a foreign plot to facilitate a conquest...

I'll shoot in the dark...

Arson. Someone is setting on fire either the cereal crops or grain silos. On the surface it could look like gang wars or factions fighting each other (sorry I don't know if your setting has medieval government style or if it's more contemporary, but you get the idea).That could be sinister on its own, but maybe the arson is the means to an end (maybe someone needs a gargantuan amount of cereal ashes for a ritual or something...). The factions could be aristocratic ( one Marquis against another) or of production type (winemakers aren't doing well, so one unscrupulous winemaker decides to play a dirty move against the beermakers).

Other sinister reasons... A mysterious disease or condition that only attacks beer makers and/or cereal farmers, if they're local. Turns out there is some mad scientist/alchemist among the beermakers, and is using the people around him as test subjects for his poisons or diseases.

Both these reasons would also come with a short supply of beer though. And if you affect cereal you would also see shortages in many other things that use cereal. Perhaps it's safer to have something affecting the beermakers themselves and not so much the cereals.

I'm not sure if this suits or what other things you would be okay with, for example, would an increase in beer demand be okay? How much magic in this? How crazy is too crazy? I guess it's easier to think of inflation as a generalized thing, not so much specific of a single product in an area.

A powerful fae has charmed the largest distributor in the city, such that every barrel of ale he sells, he charges one more piece of silver than the last barrel. So prices are steadily climbing, and everyone thinks it's just the distributor being greedy. The fae's true agenda could be whatever makes sense for your larger conspiracy.

The town's blacksmith has been coerced into cooperating with a whimsically evil group of enchanters -- any weapon or accessory they work on secretly gets a nefarious spell put on it that makes tavern keepers overcharge you. It's part of a conspiracy to crash the ale economy so the enchanters can take over the taverns and jack up the prices for real! Or... maybe that's just what they want you to think and they're actually going to enchant the ale too with a more powerful magic that only works via ingestion -- allowing the enchanters to mind control the intoxicated adventurers and get them into incredibly silly misadventures that they can watch remotely as entertainment!