It seems to me that if the aircraft was being towed around a curve, the engine thrust (or just the inertia of the aircraft if the tug were slowing) would tend to push the back end of the tug (or the one hooked to the towbar) to the outside of the curve. If the tug were too light and the rear tires didn't have enough traction to resist this force, the tug could spin partway around, in effect jackknifing. At some point in that sequence, the side of the towbar would presumably come up against something it wasn't supposed to.