The type of AU (alternate universe) where there’s no definite divergence point(s), or the point(s) are far in the past, which resulted in a bunch of canon events, people and settings being just a bit (or completely) off.
Oh those characters switched powers, this bad guy is actually a good guy, main character had a completely different childhood etc etc.
Some authors love to do that, but I hate it, one thing about fanfiction I like is reader foreknowledge, why would you read fanfics if not for the characters and events you already know.
This is just annoying, hence the name of the tag.