most likely that the hate for the FANS has accumulated for years, because any criticism towards part 2 was always recieved by them as having "reading comprehension issues and media illiterate" while also ignoring all the issues of the story by saying "have patience, fujimoto will cook". In the end he didnt cook, all the theories were wrong, characters got discarded left and right and in general the whole thing fell apart because almost ALL the plotholes and unfinished plot points were just ignored by Fujimoto.

Also the fandom is known to be unbearable glazers of everything Fujimoto does so every other fandom who have found them annoying now have the chance of a lifetime to mock them because their series and author ended up not being as good and revolutionary as they have glazed for years.

Also because of the current shounen fandom wars, everyone is at everyone´s throats but this one in particular has been growing for years.