My intuition is that this is mostly projected insecurity. For somebody who’s doing something ethically questionable, it’s pretty handy to have some group on hand to blame for being self-righteous and insufferable and so on. Very convenient indeed.

PS: ethically questionable does not necessarily mean “bad” or “wrong”, but it is undeniable that vegans and vegetarians cause less animal suffering and environmental damage than the rest of us. Anyone who can take a deep breath and look at things rationally should be able to admit that.