[abugheratwork]

They're being jerks, but why is an interesting question.

I don't think it's a cognitive dissonance thing, at least where I've seen that picture. I think it's an us/them thing. Specifically, the kind where one group gets the idea that another is out to get them. (That "militant vegetarian" you mention is someone I have heard about my whole life yet never met.) If they don't have a lot of vegetarian friends, you may be their first chance to have a conversation about it, and that's probably the only one they rehearsed.

It may not be as crystallized as other areas, yet, but I think this is also one more field for cultural proxy wars, as we all get drafted into us/them camps. Again, at the individual level, you might notice the person doing the attacking always thinks they're defending. Complicated big picture stuff at play.