I have seen this too. Don't know what it is, but it is not uncommon for someone to drop out of a run or an exercise.
My opinion is that if someone has a pre-existing undiagnosed condition, the military will find it. Sometimes fatally.
Try waking up every day at 5am for a 10 click run, with obstacles every 100 meters. Do that for 6 months to life. If you make it you make it. If you don't it's not your fault. That's just the way it is.
@240B I have been on 5 mile morning runs before breakfast wearing jump boots and fatigue uniform,and seen people break formation to puke up last night's whiskey,and then run fast to catch up and get back in their place in the column. Done it myself a time or two,too.
I have seen people just fall right over already asleep from fatigue and sleep deprivation,but I never saw anyone die from it.
The tells me the USN and the USMC are doing something wrong.