One of the commenters told everyone exactly why. Don't lock your knees.

If you stand at attention for long periods with your knees locked for some reason you will go down. We were taught that in the military and yet at every function someone would hit the floor, including me one time. kek

At least I didn't keep going until I hit face first. I just went straight down on my butt, and came right back up. The heat was unbearable in Fort Gordon.