For example, I don't believe in the existence of Big Foot, but I wouldn't start threads to make fun of people who do, I mean...why would I do that? But, if I did keep starting threads about Big Foot, maybe it is because I don't entirely disbelieve. Maybe the idea of Big Foot intrigues me, even if I don't believe. We do things for reasons, so just wondering what motivates atheists to start threads (not join threads that are started by theists, that's different) about how much they don't believe in God?
Probably because people who believe in God try to influence societies with their beliefs such that it affects others who don't believe in God or may even believe in some other god. Politics is a very obvious one, womens health, property taxes, school curriculum, science, etc.