I really don't think it's hate, in the classic sense. I think most of it is sort of a hamarotic response that's made possible by the fact that these forums show up in everybody's feed, and given that vegans typically have negative views on the eating practices of the rest of the world, can be seen referring to those people as they do in private. As you seem to be insinuating, it feels-bad-man to have your lifestyle casually attacked, and nearly always elicits retaliation because humans.

I feel like a lot of it is a matter of terminology. For instance, using the word "omnivore" instead of "carnist", or "Bovine Matchmaker", instead of "Animal Rapist".