Not really without knowing which instance you're talking about, but one guess is that you might be following a hashtag that has become more actively used.

I'm still pretty new to Mastodon, so I don't really know much about it yet. But, on top of hashtags that you follow, could it be posts that are shared (or boosted, as Mastodon seems to call it) by another user that you follow?

Aside from the hashtag hypothesis, there are multiple timelines on Mastodon: trending, local, and global for example. I'm assuming you didn't suddenly accidentally start looking at a different timeline but still worth mentioning.

Which one of the timeline do you mean exactly?