Maybe the rifle escaped from the truck of the car and followed him in to the school.
Previous lawsuits against auto manufacturers led to a requirement that a latch release be located INSIDE the trunk of every car. Any weapon that is as sentient as the Bushmaster must be would certainly have known this fact and exploited it.
Anyone here remember the Stockton Massacre in 1989? The way some of the news accounts were written, it sounded like Patrick Purdy was just standing around while his AK-variant hosed down the playground with bullets. It's almost as if the gun was floating through the air, intent on mayhem, and Purdy was desperately trying to prevent the gun from shooting those kids.