Keep in mind that the authors do admit this study to be not much more than a collection of anecdotal statements. While it's certainly interesting, be wary of using these stories to generalize about Christian deconversion.
I'm not really surprised about the reasons they found--while I do not have any serious experience with Christians, I would expect their relationship with God to be treated like any other. What good is a personal relationship if it has to be intrinsically different from any other relationship you have?
These are written from the angle that Christians deconverting is bad.
Cognitive reasons
Breaking up because a relationship with God becomes unworkable
Leaving because other Christians aren't empathetic about doubt
At what age do people leave?
It looks as though there may be more articles in the series.