His job while serving at the Balad Air Base was prison guard. The speculation is that he caught the religion from the prisoners he was guarding.
As a theory, it's pretty thin. But it definitely makes more sense than PTSD, given he was never in combat and that Balad, being the second largest US installation in Iraq, was pretty comfortable place.
Hope they requested every letter and email he wrote as part of discovery.
Mad men often become very verbal