I agree. When a mag switch is in the off position, the mags are shorted out. If one of those leads has come off for some reason, then that mag is live. Which is why mags should always be treated as live. Maybe one lead was disconnected for some reason with the vibration of flying, the disconnected lead touched something and then grounded the mag.

You nailed it, Raffles. I believe that is *exactly* what happened.

Which is why pilots leaning against propellers having their photos taken, always raises my hackles!