I think it's fair to call me an elder lemming at this point, as I recall the reasons for both. I signed up on .world with my first account on the last day before the infamous APIocalypse.

Along with myself, a lot of us were putting effort into posting good stuff, upvoting, and generally having a lot of positive interactions. Someone on !lemmyshitpost@lemmy.world posted a picture with a can of beans with a caption along the lines of "I hear that lemmy will upvote anything. Here's a picture of beans". It quickly became the most upvoted post of all time.

As lemmy culture developed into a deeply political space, it began flooding !lemmyshitpost@lemmy.world with American politics. People got sick of it, and people started doing "we're posting x now until politics stops". There had been a couple rounds of various themes, and then someone announced "that's it, I'm sick of politics everywhere. We're posting corn". Cornposting has become the default political purge over there.