Short version: yesterday, Lemmy.world announced essentially, though they buried it under lots of distracting verbiage, that they were going to follow in the footsteps of Vichy Twitter and Meta and allow hate speech and misinformation, and probably more to the point, they intend to force recalcitrant mods to allow it regardless of their own views on the matter.
And the head mod/creator/whatever they are of unpopular opinion took exception to that and locked the community in protest.
From the announcement in question: