I had a friend that dumped a 105MM howitzer from an H-34 and several days later he dumped the Generals Jeep from about 1000 feet. The problem was traced to a small Nylon ® pin that was employed as a striker on a Micro Switch ®. This striker wore and eventually kept the low weight micro switch in the closed position. This armed the low weight micro switch so that when the pilot armed the hook it appeared that the weight was off the hook and it opened.
Static electricity was mentioned, as was high voltage radiation as a possible cause. If solenoids are employed in the hook circuits or a solenoid was operated while the hook was armed this could be a problem. When a solenoid is de energized and it is not protected it can generate a back electromotive force as high as 1400 volts. This high voltage can radiate and be picked up on the circuit for another solenoid and cause it to operate. This EMF can also effect other circuitry. The back EMF can cause switches to weld shut or to severely increase the resistance on switch points causing a sneak circuit or no circuit at all.
Edited to remove a zero.