I think Henry's closest to the mark. This article
gives these figures:
Soldiers between the ages of 18 and 24 accounted for 45 percent of the deaths, and 54 percent were among personnel of low rank. About two-thirds had been deployed in active combat.
Only a little more than half were low ranking, meaning there's a healthy percentage of officers killing themselves. And a full third of the whole group didn't even go into combat.
My vote for chief cause is the multiple deployments with no end in sight. Not only do they keep sending them back, but there's no "X days and a wakeup" to keep them grounded. It just never ends.
Lots of secondary causes to choose from but if you put a healthy human in a toxic environment long enough with no end in sight, no matter how well you try to treat them, permanent damage results.