The connection between atheism and communism in the public imagination, at least in the US, would be boring if it were not so common, and hence something that needs to be dealt with. At least some of the hostility towards atheists is based on hostility to communism (or to its specific Marxist-Leninist version, or to its manifestation under Stalin, or to the totalitarianism and militarism of the late, unlamented Soviet Union).
It's not only in the US that this connection gets made, and in all honesty we should acknowledge that some (perhaps many) people did adopt a commitment to communist politics and an atheistic view of the world all as part of a package deal. Alister McGrath seems to have been one of those people in his younger days, as he discusses in The Twilight of Atheism. He says (p. 176), "The principal cause of my atheism, was Marxism, a movement that I believed held the key to the future." On the following page he adds: "Let me stress this point: the appeal of atheism for me lay in its proposal to eradicate religion."
More generally, McGrath seems to think that to be a true atheist you have to swallow an entire atheistic worldview, including some sort of political anti-theism.
So, I don't doubt that there are people who once were, and others who still are, like that. But McGrath needs to understand that his experience is not typical or generalisable. The existence of people like the younger McGrath is a long way from establishing the myth that "Atheists are communists." Many of us have no great love for communism or any other comprehensive political ideology. As far as I'm concerned, once I freed myself from Christianity, after a considerable struggle that I talk about in 50 Voices of Disbelief, there was no damn way I was going to take up another religion (one of the versions of Marxism or communism on offer) in its place.
I grant that Marxist-Leninist communism might not technically be a religion in the sense of positing a supernatural or otherworldly order (whatever those words actually mean when you analyse them). But it apes Christianity in offering a totalising view of reality, holy books (The Communist Manifesto and Das Kapital), prophetic leaders, deep psychological transformations of its so-called "true believers", extensive canons of conduct that people are expected to follow, and an apocalyptic vision of history. Indeed, it effectively portrays History as its God - participation in the movement involves having History on your side with the prophecy of a kind of earthly paradise at the end in the form of the communist state (which follows the earlier stage of a dictatorship of the proletariat). Having History on your side can legitimise atrocities much as can having God on your side.
While this may not technically be a religion, it apes religion so closely and comprehensively that it looks very much the same and plays much the same personal and social role. It is just as dangerous as any supernaturalist religion, and merits just as much criticism.
Many of us are atheists for reasons that are remote from any of this. And far from wanting to eradicate religion some of us were not especially inclined to speak up in public about these issues until relatively recently. Not that many years ago, I was writing media tie-in novels and doing most of my "serious" thinking and writing in the field of philosophical bioethics. I was actually pretty happy with that combination (though I freely admit that I've always maintained some interest in philosophy of religion).
In 50 Voices of Disbelief, several of us talk about our reasons for speaking up about religion at this point in history. In some cases, including mine, those reasons are, indeed, broadly political. But even when they are political, they need have nothing to do with another totalizing system such as some version or other of Marxist-Leninist communism.