Alternator Issue

I was doing a valve adjustment on my 90 caprice and when removing the driver side valve cover, the alternator to starter wire arched to the valve cover. After doing the adjustment I noticed the car was running on only about 9 volts. I checked the wire that goes to the battery terminal on the starter and it had broken from the current. Easy fix however when testing the terminal on the back of the starter, it was showing ground when the car was not running. I figured I may have blown a diode but when I got the starter tested it passed with flying colors. I'm ordering a new alternator anyways but I'd like to know what could have possibly been causing the alternator to ground yet also pass a bench test. Is the housing possibly grounded to the bracket and something that would normally be insulating the stator possibly "fried" and closing the circuit? thank you for your help. Matt M 602-616-0390