I suspect a broken firing pin. Years ago, I had a SPECTRE 9mm semi-auto go full-auto on me at the gun club for precisely that reason; the firing pin had broken and was jammed in the bolt sticking out just enough to set off the cartridges as they were being chambered. I remember loading the magazine into the gun, pulling back the bolt, releasing it and then BRAAAP! Finger was nowhere near the trigger, and fortunately the gun was pointed in a safe direction - two rounds through the bench, and three more rounds in the floor downrange before it jammed (it was a 30 round magazine.) The next day I brought the gun back and traded it in for a nice safe revolver lol.