Yeah, I didn't start liking spicy food until I was a teenager/adult and when I started eating it regularly there was a bit of a reckoning until my body adapted.

That said I think it's a combination of spicy and lots of beans. If you don't get enough fiber in your daily diet your meal every few weeks that includes an appropriate amount will cause some gastrointestinal discomfort.

Beans and capsaicin. If your daily diet is low fiber low spiciness a meal that's spicy and high fiber will do that to you.

For comparison consider the "traditional" diet of the American Midwest. It's largely casseroles from canned goods, with large amounts of dairy. The plants involved are often pasta, potatoes, carrots, or canned. You also have cut of meat + fried potato. It's a diet high in sodium and fat and low in fiber and where piquancy is delivered in a dash of pre ground pepper. It's the sort of diet you'd recommend to someone who would rather schedule a day for their bowel movements rather than spending less time more frequently (and easily) on the task. This is rarely how people eat every day even there anymore, and it's an extreme example, but it's not that far off of (white) poverty food in much of the US.

To someone with a diet even resembling what I just described, a dish with lots of beans, some rice, veggies, and some kick to it can very easily overwhelm your digestive system.

Then you add some racism in to generalize it in a way associated with food in poorer countries, and add in the fact that Mexico is the most common place for Americans to get travelers diarrhea (often called "Montezuma's revenge" here).

All these issues are really easy to blend together in your head if you're the sort of person who normally eats like crap and doesn't do research before taking a vacation to Mexico.