Atheists do not believe in supreme beings commonly referred to as gods. Vegans do not believe other-than-human animals exist merely to serve human needs. That is, vegans believe nonhuman animals exist for the same reasons we do—their own.
Is there a common bond between atheism and veganism? I believe so and I suggest it is doubt. That is, in a society such as here in the USA where “more than 9 in 10 Americans continue to believe in God“, doubt is at the heart of what helps turn a believer or likely believer into a non-believer.
From my perspective, the same goes for veganism. In a world where other-than-humans are routinely thought of as things to be bought, sold, used, and possibly even killed for mostly trivial human needs, folks who think differently are a rarity. Once again, doubt, I believe, is key to acquiring this currently uncommon worldview.