the cop lost it. Witnesses (multiple witnesses of which one did not come from the area and did not know Brown and was right behind the cops car) said the the cop was trying to pull him into the car through the window and Brown was resisting. Brown did not put his hands in the car and did not initiate contact as it was the officer who did that. The gun went off once inside the car and that's when Brown broke free and started to run. The cop exited his car and started to fire and one bullet struck Brown in the back. Brown stopped and put his hands up and the cop then fired several more bullets into Brown.