I think it's a multi-part phenomenon:
Post Vietnam, the US has been involved in many conflicts - but other than GWI, none have approached the scale and intensity of Vietnam as GWII and Afghanistan (extended deployments, number of troops involved, ferocity of combat, environment/location of combat etc.) Even GWI is difficult to include in the same category - sure there were many troops involved in extended combat and that conflict certainly lasted long... but nowhere near the commitment levels of GWII and Afghanistan.
A large part of it, I'm convinced - is that more soldiers today LIVE post traumatic incidents than in wars past. To but it bluntly - the increase in numbers of veterans claiming benefits is because we have more soldiers surviving battlefield injury.
It's got to do a lot with advances in modern medicine (anyone who has been a nurse for over 10 years knows what I'm talking about) and a 'change in attitude' from the 'powers that be' with regard to casualties on the battlefield - while battlefield triage still exists, cas-evac and availability/location of specialized medical care have dramatically improved to the point where soldiers today survive often fatal injuries in the past. One just needs to read any factual book on the Eastern Front in WWII or a combat medic's record in Vietnam to get an idea.
Part of it also has to do with the KIND of war the US has fought since Vietnam (where we experienced an inkling of it - in Vietnam, we had the VietCong but also the NVA. Today, it's mostly just irregulars like the VietCong). Most conflict these days is asymmetrical.
Troops live amongst and police the 'conquered foe'. Or they are fighting "the enemy" - but it's often hard if not impossible to discern WHO the enemy is. And given the locales we're fighting in - it's near impossible. The friendly farmer you talked to a few hours ago might well be the chap heading a group of his fellows launching mortars at you by night...
This places an enormous burden on the average private - you can't tell who is friend and who is foe. And it's not just for one day or one week - it's months on end. And then, you have multiple deployments.
So here you are, far away from family and friends, amongst a hostile populace, instructed and ordered to do things you'd usually find objectionable, you're here for months and perhaps years on end; well, what else is supposed to be the outcome?