Some people feel judged for eating meat if they learn that you’re a vegetarian or vegan. As veganism appears more extreme, it causes a stronger reaction.
These people don’t like being judged, so they seek to judge you instead. If they can judge you for not eating meat, you can’t judge them for eating it, or so they feel.
Some vegans being quite obnoxious in their attempts to convert others, doesn’t improve things. Rather, it helps those who dislike vegans to reaffirm their beliefs that all vegans are silently judging them all the time.
All of this is just my theory.