It is beginning to look more complicated than a simple oversight by the crew. Possibly that something had been spotted during the pushback, and whilst investigating this, something in the steering gear failed whilst under load... The subsequent tension release being what has caused the banksman to become pinned.
All speculation at the moment, as it's such an unusual type of incident... The towbar latch shear pin is the weak link when towing, and it is not uncommon for them to snap if the aircraft is jolted whilst towing forwards... But when pushing back, the loads are reversed and this doesn't happen.
Suffice to say that both engineers and the AAIB have spent a large part of the evening working on and around the aircraft. It was finally towed, very slowly, clear at about 22:30.
Be interesting to read the report when it is released, although it wouldn't be the first A320 family aircraft to suffer nose gear steering issues though...
No word yet on the chap concerned, but it will undoubtedly be a long night for him and his family.