I think it's because a good number of vegans are, shall we say, a little bit too vocal and judgemental about those that are not vegan.
If someone tells me they're vegan, and I say cool, I do eat meat, and he responds with "you're a murderer" then you are bound to get people looking weird at you
I wouldn't call it "a good number." It's a very small minority. Outside of the memery of the militant vegan, I've never experienced a vegan preaching to me about dietary habits.