I'd bet that's true, partially because of alienation by conservatives. Why should atheists side with a group who considers atheists to be second-class citizens, somewhat in the way Muslims look at minority Christians? But there's nothing inherent in atheism that points to liberalism. The seminal atheist work Atlas Shrugged isn't exactly the liberals' Bible.
It is true that the vast majority of atheists are liberals, it is natural for someone that does not believe in God to reject so much of conservatism, that only a relatively small number of atheists that like conservative economics could bring themselves to join with conservatives and traditional Americans in voting or politics, even then they are always badgering us to move left.