"Push a vegan agenda" sounds evil, sounds like they're trying to convert others by force, etc. And at the very least it definitely betrays your thoughts on the matter, showing disdain for the cause. It's a biased type of statement so it's hard for everyone to agree with you that "that's what it is."

Calling it "Vegan activisim” is a more neutral term that probably wouldn't get you down voted. Doesn't really betray what you think of it one way or another either.