It would start making sense to you, if you’d see the analogy in racism et al (unless of course you are one, then it won’t). An animal-rights-activist-vegan sees it that way and hence has a hard time to “shut up” about it. Like you would when you’d enter some nazi-meeting. Can’t just sit there, doing nothing, and thus invoking the feeling you’re part of it.

It’s not vegans per se, it’s those that are just vegans as a direcr consequence, not those that follow a dietary decision.