It was probably the frangible link in the tow bar breaking (which does usually happen if the crew apply brakes while being pushed, or the truck tries to push with the park brake still set.). It can also just let go because it's getting a bit old. Quick inspection to see if the nose gear was hit by the tow bar, and off you go. (After all, it's specifically designed to break in order to prevent damage to the aircraft.) Not usual, but not unknown, either.
Move along, nothing to see here.