I jumped the theist fence after reading a book whose intellectual force was too great to be denied outright, and too difficult to refute point by point. I hate being wrong, and feeling stupid, and the arguments from the book stayed in my thoughts for a long time.

I didn't formalize my thoughts until later, but if my atheism had a cause, it was THE CASE AGAINST GOD by George H Smith. I was very emotionally satisfied with my religion and it's community beforehand.