Nine armed servicemen? And they let themselves be disarmed and slaughtered? Something’s wrong here.
When all is said and done, I suspect it will boil down to an ROE-induced case of gunshyness. But don't expect that to ever be communicated outside of certain circles.
All I can think of is that at some point during or after an argument the Afghan got behind a machine gun and ordered them to drop their weapons. Having known the Afghan long enough as a student, they probably convinced themselves that he was just angry and wouldn't actually shoot, and were complying to help defuse the situation. Maybe after taking one hostage, the others decided to disarm to talk the Afghan out of it?
Either way, once they were disarmed, he simply mowed them all down, then waited for the QRF. That's just speculation, but I don't see how else you could feasibly kill nine people before one of them rushed you. Most everyone I knew in Afghanistan had a knife handy for exactly such business.
I'm also against disarming at gunpoint with ten to one odds in my favor. The military is extremely risk averse these days, and a lot of that is ingrained more and more into basic decision making. There are still holdouts of aggressive thinking and action, but the military culture is increasingly one of passivity and ass-covering.
Back to the incident. Unfortunately, literally everyone who could shed light on this is dead. There'll be some forensic information of value, but no real explanation of the incident itself. What the troops were thinking is anybody's guess now.