I have a similar background ... military, moves, desert .... most people will have a bad day. Dehydration, sleep deprivation, over-exertion, over-stimulation, emotional swings, almost any reason. I hit the wall around Wednesday or Thursday morning. I nap, eat, drink water, fuck, and then I'm fine. Not in that order. So if you're out there having a blast and suddenly hate the world, don't be disappointed in Burning Man, give yourself a break and a rest.
This is discussed on the burning virgins tribe. Happens more often than people recall; the good times outweigh the bad and not many think to share the negative few hours.