I'm not a vegan, but I don't see why this is particularly surprising.
I'm of the view that the only notable considerations for being vegan are moral ones. Mainly, environmental and prevention of suffering from animals. To them, (and arguably also to everyone else) it is very important not to hurt animals, not to needlessly kill them since they view meat eating as a luxury and not a necessity.
In this sense, it's not necessarily just your right to eat meat or your right to make your own decisions. We aren't allowed to kill humans on the basis of freedom, and in a similar vein you can see why vegans don't want people to kill animals, and hence why they might project their own beliefs on others. If you believed that animal suffering was of near or equal importance to human suffering, then it seems perfectly reasonable, almost expected to stand out against animal suffering in the way we stand out against human suffering. Preachy, sure, but I think justifiable at the same time.
Saying this, vegans should be careful to assume that they have the complete moral highground. Veganism ultimately still kills animals (on a much lesser degree of course), so they shouldn't assume that their actions are entirely damage free.