I think atheism gets called a religion because atheism has become more than a belief in a certain sense, particularly in the West. It's turned into a movement of sorts as many vocal atheists seem to have a shared collection of beliefs about theism, religion, religious people and the world, in general. When people see that, they feel the need to label it as something and religion seems like an apt label given the subjects that prominent atheists and atheist movements tend to concern themselves with. However, I would just say that it's a movement rather than a religion. That's more accurate.