I think it is because they were not psychologically prepared for the job. One who chooses to join military must be both physically, mentally and psychologically prepared and ready. This is because there is a lot they will encounter and they will have to know how to deal with.
idk for sure but i do know that the military requires a huge lot of strength, discipline and resistance , for some people this ain't easy at all. maybe they jut feel over-worked (if you know what i mean) or they can't stand it.