Slick - the towbar is the " weak link " between the NLG and the tractor. Shear pins are rated to take into account breakaway and drawbar forces. Although tractors have been known to punch gears up through electronics bays it would be an extreme and non normal set of events such as brakes parked and excessive force applied by a tractor. I read an article many years ago where a B707 collapsed on a tractor, killing the driver. He was believed to have suffered a heart attack and collapsed in such a way that he applied maximum accelerator with the aircraft brakes set.