I would hazard a guess and say maybe, just maybe cables within the door may have chaffed to the point where wires for the wiper motor and door lock mechanism may have bridged. Each time you unlock the vehicle and that voltage pulse is sent to the various lock motors in the doors, some of it may be feeding into the wiper's circuit in the tailgate, hence the brief movement you're observing.
Try pulling off the door card on the tailgate and look around for anything out of the ordinary with the cabling.