If the brakes were set, it's unlikely that the aircraft would move, especially with a tug still attached, therefore it's a reasonable assumption that someone had deliberately released them. That is generally done when you want to taxi away...
The only other scenario I can think of is the tug driver not asking for the brakes to be set at the end of pushback and a crossbleed start needing more power on the left engine. Not incredibly likely plus the aircraft would have to travel a fair distance to impact the tug, if it started from the normal towing position - one would have hoped someone would have noticed something amiss before a collision occurred.
You can rotate the 777 around one main gear with lots of power for a minimum radius turn in extremis but it needs the brakes held on one side and released on the other otherwise it's not going anywhere.
Edit: One other possibility - the towbar sheared with the engines running and the aeroplane overran the tug?
Last edited by FullWings; 5th May 2013 at 01:43.