just looking at the pictures of the tug and the wording of the press release. you can see the towbar is still attached to the tug and the tug is at a right angle to the aircraft.
I believe that the training for a tug driver says in the event of a separation you should accelerate out of the way of the aircraft as fast as you can and that it is the responsibility of the person on the aircraft brakes to stop the aircraft. if I am correct then one possible scenario is a towbar separation which was noticed by the tug which then turned sharp left to get out of the way, the brakeman however did not notice the separation or was not able to stop the aircraft in time and effectively ran the tug over.
and before anyone asks it is not that common for the tug to be in contact with the aircraft during a towing operation.