I've had this happen to me with Shu! where the end stage planet gears just got stripped. It wasn't that the motor was being overvolted (it was a 12V motor running on 9.9v), it was the load on the end of the lifting arm which then got stuck as the arm tried to continue moving. Torsion did the rest I should think. I seem to recall the gears being brass, which yours does too.