My guess would be that the regulator stuck when you turned it off. With current going back into the generator, it tried to "motor" and of course couldn't. And something is going to burn until you have a open. Either the regulator, wires or the gen. This is why we always installed fuses of about twice the output of the gen inline with the gen wire. It was not that an uncommon occurrence. In addition to installing fuses, I was always tough to look at the ammeter when I turned an engine. We wanted to avoid a fire in a gasoline delivery truck.