as we are looking at a repositioning of an empty aircraft, overstressing the shear pins is unlikely, unless in a very tight turn. then both the torque and axial pins would have to shear beafore the tow head detached from the bar. more likely that the pins were damaged by uneaven aprons which can cause the head to splay, breaking all pins, which may not become apparent until the tug turns and the head comes under pressure. BA have a flag that springs up to indicate a pin failure, but this can usually only be seen when pushing back with the driver facing the aircraft. when towing, this flag is not normally a help.
groundhog: pins should be inspected for wear/damage at least at 90 day intervals, but i know many are not. as you say, you just wait until they break.