DBS explains it well in his new post. Excerpt:
There is a very tangible change which happens some time after you have left religion and have had a chance to reacclimate to the world. At some point, you look back at the belief system which you left behind and feel a sense of shock at what you see.
This may really be the point of no return. Up until then, there is a sort of built in defensiveness in your thinking. You have all of your reasons – logical and moral, all worked out in your mind - as if you have to justify your choice to leave the Orthodox world. But at that moment, you suddenly grasp that the shoe belongs firmly on the other foot. You have the powerful feeling of seeing, for the first time, your old beliefs on equal footing with the claims of the other religious groups.
And, just as suddenly, your need to justify your ideas evaporates. “Am I really concerned about explaining why I don’t believe in this outrageous mythology?” “Am I really worried about proving that I’m still moral?” You feel, for the first time, that it would be just as absurd to have to justify why you are not a Mormon or Scientologies.
From this side of the shift, I cannot understand how smart, educated people continue to believe in Orthodox Judaism. I know that sounds arrogant, but it's true. I literally cannot understand it. I've come up with various hypotheses ranging from psychological mechanisms to the idea that a lot of them are just faking it, but I don't get it at the gut level as I once did.