I think that if you really look into those with PTSD, you'll find that a great amount is due to cumulative trauma, even in those who have never served.

Many people experience horrific things in childhood and are able to heal, only to be re-traumatized. The childhood trauma may not cause full blown PTSD but it does lay the foundation for future problems.

I think there's a great danger in only assuming that the most current trauma is responsible for the disorder. In reality, all of it needs to be worked through.