For military analysts, the reason is the nightmarish experience of sustained combat: Soldiers have been fighting the longest war in U.S. history, with frequent stressful deployments and compressed rest time back home.
You can't keep taking people away from their family and friends, inflict long deployments with many opportunities to be hurt or killed, and expect them NOT to show signs of stress.
I've written before about being deployed over the years missing birthdays, anniversaries, graduations, births, etc. Troops are not the same when they return, it takes months to re-assimilate back into the home (if they can). Knowing that they will have to do it all over again in 18 months makes it worse than anything WWII or Vietnam vets faced.