Brass pickers. Maybe they were poaching some shooter's brass behind his back, he caught them and went ballistic. It isn't impossible but unlikely. However, I've had people at NRA High Power Service Rifle matches poach my brass while I was down range pulling a target. I've never heard of people getting in fights over brass picking territorial rights but I suppose it's happened. "Hey, I come here all the time. This is my place." Anything you can think of involving human behavior has probably already happened at least once.