Shit happens. It is apt to reflect headlines in some way, but unless you are capable of living inside the shooter's mind I think it silly to speculate on causes in such case.
Social causes (that can be grasped in analysis) operate at much more abstract levels.
Military training does _not_ include (for the vast majority) much training in marksmanship. Only a very few soldiers are trained as snipers. There are probably more expert marksmen trained in hunting, etc. or in shooting clubs than in the military. My wife's father is an excellent marksman, _and_ he was in the Marines (at Iwo) -- but he learned to shoot as a teenager, hunting, not as a Marine, where his task was primarily to carry around belts of machine gun ammunition, which doesn't do much for accuracy as a rifleman.
Carrol