For me it’s the high-horse holier than thou attitude most of them seem to carry in online conversations. I know a fee vegans and they are mostly fine in person after the first few months of radicalization, but I imagine they just suppress it in person to maintain the acquaintanceship and then bitch in their vegan echo chambers about how “my co-worker who knows I’m vegan had the audacity to order a hamburger and eat it in front of me knowing I’m vegan, does he know he’s destroying the world with that Burger… AITA?”

If you’re looking for scientific answers, good luck they, Inrhjnjbmost people stop worrying about micromanaging people after a few years of academia.