Likely it was a shear pin that broke. There are typically shear pins on either end of the tow bar, tug end and aircraft end. They are there as a sacrificial piece in case the loads are too much on either the tug or gear to prevent damage. Can happen for a number of reasons. SOP when one breaks, no matter which end, is to inspect the gear. I broke one once when repositioning a -700 to a hard stand and used too much brake on the tug and the empty weight of the aircraft put too much force on the shear pin and it broke.