There's not much in the system:
CB, control panel and combined gearbox/motor/control circuit (behind the wiper)
The CB provides power to the wiper motor, but a second variable voltage supplied by the control panel to the the wiper motor assembly controls PARK/INT/LOW/HIGH.
For some reason, a stray control voltage is getting to the motor assembly. There are a few diodes in the control panel, maybe they were breaking down and affecting the output voltages from the control panel. Or maybe the contacts of the switch were faulty. The panel is the easiest to change. Given that the fault was very intermittent and unlikely to be faulted on the ground, the engineers would have probably changed that as a first course of action.