Though most often attributed to the difficulties experienced in combat, or the difficulties in reintegrating to peace, these problems are often prevalent in non-combat related units as well. Why might this be?
One theory may be that it is a "self-fulfilling prophecy." That is, soldiers drink, use drugs, get in fights, and drive their cars off bridges primarily because that is what is expected of them. When they return home, the previous social rules that caused them to stem their bad behavior no longer apply. People give them brakes since they are vets, and allow them to be loud, not work, sleep in, start fights, and everything else. So they do it.
Though this is obviously not true for many, it may be the answer to why for some.