Ground crew clearance received?
The aircraft collided with the tug after it had been disconnected following push back. The aircraft either rolled forward or commenced taxiing before the ground crew and tug were clear.
This sort of incident has happened in the past at BA and it is drummed into us not to request taxi clearance until both the tug and headset operator are clear and flaps deployed to T/O setting.
It's all on the checklist........