The vegans who act this way are no different than the anarchists who are always screaming at "statists" and calling them sheeple. These are people who are almost always quite emotionally immature, and tend to fall into one of two categories:

1. They still hold judgments about themselves for having been on the other side of the fence for so long. As long as they are holding that self-judgment, that self-hate, it spews out at other people, because they've now changed the behavior, so those emotions couldn't be coming up about themself right?
2. These are the people who have always treated others poorly when they "know" something that the other person doesn't. People who like to tell others about something not to empower or educate them, but to show how much more they know.

Something that applies to both categories equally, as well as vegans & anarchists equally, is that these are people who do not understand the importance of communication, do not understand how much better compassion & listening are to helping someone change themselves, and possibly who do not actually care about the ideas that they espouse, only about the superiority it allows them to feel.