What do you think happened? How about this?
1. The guy was a 63-year-old Open Carry protester. The cops were unprepared for someone like him, being in their twenties and inexperienced in dealing with gun rights extremists. At the first movement they blew him away and naturally claimed he'd aimed the gun at them.
2. The guy was depressed because of the ecomony and maybe his wife just left him. Knowing the cops would be called, he went for a walk with his shotgun in hand. When the cops approached he shouldered the weapon.