I think the atheism occurs because 1, Mormon god is an asshole. 2, Mormon holy spirit is a wimp that can't even stay awake after 11 PM Rexburg, Idaho time or even watch Saving Private Ryan (or Saving Ryan's Privates) with without being offended and leaving you on your own until you confess touching your little factory. 3, Cheesy grinning attractive Caucasian white and delightsome Mormon Jesus just isn't the same guy in the bible.

So those are the 3 versions of deity that I would argue most exmormons reject on the way out. After rejecting those, the critical eye is then turned to mainstream Christianity. Maybe you could be Christian if Mormonism is a farce you think. You find that the God of the old testament is an asshole too and Jesus is a bit less of a solid character and more often misquoted than you were raised to believe.
This is the turning point where some exmormons decide they are ok with that and go on to be Christians. Others, keep researching the holes in Christianity and are then caught up and transfigured by light knowledge into great atheists with morals. The .1% or less who join non-Christian religions after a lifetime of learning they are wrong at church or something defy the odds, because who the hell believes that weird shit when you are raised Mormon with perfectly normal things like masonic temple ceremonies, baptisms for the dead, magical translators, angels, 3 nephites, Cain walking the earth, Satan controlling water, and men having lots of wives.

For me, the critical thought processes that led me out of Mormonism, tore the shit out of every other religion too. Leaving me godless and really fucking happy about it. There was also the issue of investment with me. There is not a chance in hell I will ever be duped into spending so much of my energy, my heart and soul, and my time into a falsehood ever again. Fool me once...shame on you. There is no fucking way I'll be fooled twice.