AA SLF
So - what circumstances could have caused this event?
Brake failure
Failure to "hold" the brakes while unhooking the tug
Beginning the taxi before "clearing" the taxi path
How about (incorrect) wave off from the ground crew indicating the tug was clear? You can't see a tug under the nose on a 757. FYI the ground crew member is responsible for the a/c not hitting anything during push back until such time as the captain tells him he is clear to disconnect and asks him for wave-off and display of the pin.
Also, FYI the brakes aren't "held" on by the pilots during uncoupling.......the parking brake is set to park and the ground crew are informed of this before permission is given to uncouple the tug.
I don't know what happened, but there for the grace of God go all of us.....have you never made a mistake whilst parking your car and bumped the one behind? Sometimes these things just happen, even to the best of us.
PP