American Soldier who had been acting weird lately. Being reported that he wasn't going to go along when they rolled out.
FWIW, reporter on CNN (Jamie McIntire, I think and probably misspelled) just informed viewers of the term for this type of behavior is fragging.
And in fairness he did say that now it's been determined to be a U.S. soldier, now remains the question of motivation.
Then he ticked off three possible reasons:
Subordinate unhappy with commanding officer
Political motivation, or
Terrorism