From what my uncle told me, having served for a number of years, with every batch of fresh recruits they get in the summer it is almost a guarantee that at least one or two of them will basically lock their joints while standing, thinking this is a good way to be perfectly still while at attention. However, this apparently has a significant negative impact on circulation, creating a situation where there is basically insufficient return to the heart and upper body. When combined with the intense heat that is often seen in the southwest/southern states during the summer, and the rapid onset/progression of symptoms, there is practically no chance the recruit will realize something is wrong before its too late. They suddenly blackout and end up face first in the dirt (the old "go to ground" physiological response when one has insufficient oxygenation of the CNS). He said they usually come to after a minute or two wondering what the **** is going on, why they are flat out on the ground, and how the **** did they get there in the first place.
I'd imagine something quite similar happened to that soldier in the picture above, probably having something to do with that sweltering nature of the hat and dress uniform he was wearing. That's my best guess anyway.