Some believe that most atheists deep down actually believe in God, (even though everyone who makes such claims often tends to also think that they believe in their particular God), but have just rejected God out of rebellion, spite, fear, or because of some childhood religious incident. In fact, there might be some hidden truth to this theory, but not with every atheist of course.
There have been some reported cases, such as with Voltaire and Freud, where the very strict and severe Christian childhood social and family environment forced them to the side of atheism. But on the other hand there are many atheists who didn’t have a bad experience at all. Conversely, they have nothing but good memories, from their grandmother taking them to church, to the sunny Sunday mornings in the park after church and so on. Maybe they simply started to question and to wonder what happened when they applied the standards of logic, reason and burden of proof of religion, as they already did with everything in their lives. And let’s not forget all the people who were never believers even from the beginning and who just never cared to believe in something sacred and superior. It can happen too.