My bet is that the steering lockpin fell out during the tow. Can't tell from the picture, but I would bet that the towbar head is still attached to the airplane. For those that aren't familiar, the 747 steering pin has a spring-loaded ball about halfway down the pin length to lock it in place, but they have fallen out before. If the ground handler used a 777 pin instead (same diameter, but the locking ball is at the end) you could expect a similar result as this. UA modified their 747 fleet to use a longer pin that drops in from the top, as gravity is your friend, but most other operators left theirs unmodded to insert the pin from below.
If the pin drops out during a tow, the next turn you make with the tug will have the hydraulic pressure centering the gear, and the shear bolts will break, leaving the tow head on the aircraft and the tug/bar combination wandering free. If the tug is turning, it will continue, and the aircraft will go straight until it piles into the tug, generally with the results seen here.