The cover could possibly be shorting out to the hot wire inside the cover. Maybe the insulation tape or shield around the lead wire has gotten old and fallen off. If the pickups gets moved or squeezed, the wire could be shorting out. It would be easy to repair by carefully removing the cover and checking for any stray wires or solder that could be sticking through the insulation tape. The mounting plate on the humbucker is normally grounded and if the hot or positive wire touch the cover or plate, the pickup will short out.