From the NYT: "A man whose office is near the site of the shooting said it was an area where buses load arriving passengers. Speaking on condition of anonymity to protect his business, he said witnesses told him that the gunman first talked to the military personnel to find out who they were and then opened fire, shouting “God is great” in Arabic."
I knew it couldn’t be a service member because they’re not allowed to carry their weapons on base. Thus it had to be somebody from outside. So I guess the “argument” was him screaming that hideous Islamic cry while trying to get on the bus.
I read that one of the people killed was shot outside the bus, so I guess once he knew they were military he just killed whoever he could get in his sights.