Essay:Causes of Atheism
The main cause of Christians turning into Atheists is a close reading of the bible.
Other, more minor causes include: having a basic understanding of science, especially biology and geology; frequenting websites of ill repute such as this one; having an analytical or critical mind; spending five minutes listening to Fred Phelps, Jerry Falwell, Andy Schlafly or other crazies.
For Jews, the main cause is the realization that their religion idealizes a god who killed Egyptian babies to show their unelected leader how awesome he (god, natch) was. 
Some neo-pagans or Wiccans become atheists when none of their spells work.
Others "turn the other cheek" due to bad experiences with holy rollers.
Some have analyzed Pascal's Wager to the point that they realize he assumed religion does not interfere with life, and that the odds of selecting one of the infinite amount of possible gods who reward believers over the infinite amount of posssible gods who punish believers, infinite amount of possible gods who do not reward believers (i.e. no reward at all), or the infinite possibilities of no god at all.