i have no idea.
in my opinion (i could be wittgensteinianly mistaken) i did not choose to believe in god. in do not choose to continue to believe in god. if i had a choice, i would not. but this is not the same "not choosing" that you're talking about. not having a choice and accepting evidence are very different ideas.
lucky for me, i was raised a calvinist as someone mentioned, so this at least makes sense to me a little. i am compelled. maybe it's actually god, maybe it's crazy wiring that we evolved to help promote social controls to protect offspring. either way works for me.
i don't think atheists choose not to believe. i think they may choose to stop asking questions when the questions regarding god don't produce any results for them. is that the same thing? no, because the religious tend to not like people who ask questions.