The ground at the front left of the engine bay is disconnected - factory setup is a ring terminal on a 10mm screw going into the back of the grille.
That ground services the left front marker light, the washer motor, and the horn - I suspect with the ground disconnected, it manages to backfeed through the washer motor, and since the washer switch now sees 12V, it triggers the washer cycle on the wipers as if you had bumped that stalk down.