I'd believe that. Spent 20MM cases don't have normal firing pin strikes on their primers, just a tiny pin prick because the M61 is electrically fired; current passes through the firing pin to the primer.
The primer consists of an open-ended brass cup that contains a brass button insulated from the cup by a plastic liner. The firing pin of the gun contacts this button. The ignition charge (a conductive explosive mixture) is in contact with the other side of the button, and is retained by a paper disk and a metal support cup. The electrically initiated primer ignites the propellant charge. The primer explosive element is sensitive to electromagnetic and electrostatic energy.
I suppose it could have been set off by static discharge.