I've come to believe that those claiming atheism is a religion are either threatened by the thought of someone living happily without religion, and therefore have to classify it as such, or have a willful ignorance of working definitions.
You don't have to "believe" in evolution, the Big Bang, read Dawkins or Darwin or Hitchens or any so-called "atheist books" in order to be an atheist. All you have to do is say, "I don't believe god(s) exist." The end. There simply isn't anything else to it than that. Sure, there are communist atheists, Buddhist atheists, and everything else KC mentioned. But saying this is symptomatic of religion is to clump Protestants with Catholics, or Christians with Islam, or Taoists with Deists. By the same "atheism is a religion because X, Y, and Z atheists do have some form of religion," then all Christians people must believe exactly the same thing, right? Except that's not the way it is. In fact, it's the exact opposite of the way it is. Some Christians believe accepting Christ is enough. Others, like the Church of Christ where I was raised, believe you HAVE to be a member of the Church of Christ to get into Heaven. Sorry, Baptists, Methodists, Presbyterians, Anglicans, Lutherans, Anabaptists, Pentecostals, Catholics, and so on. But being Christians does not necessarily mean you believe this, just as being an atheist does not necessarily mean you subscribe to the beliefs of X, Y, and Z.
Just because elephants are gray does not mean all gray things are elephants. I know it, and everyone reading this post knows it. Not all Christians adhere to what the Church of Christ preaches, and we plain atheists don't necessarily subscribe to the beliefs of other atheist groups or organizations just because we are atheists.
Yet no matter how many times this is explained, staunch "atheism has to be a religion!" advocates seem to never falter from their comfort zone of assuming all peoples have to have religion, no matter what it's called. Sorry, it doesn't work that way. Do Christians make their atheism of Zeus a religion? If atheism is a religion, does that mean Christians hold many religions, as they disbelief in the existence of gods all except theirs? Of course not. Nor does my disbelief in Zeus constitute a religion anymore than my disbelief of the Christian deities.