this has happend while the aircraft was stationary , the nose gear has retracted into the bay , as the nose gear doors are open it would seem that the hydrulic power was ON as this is needed to move the doors (except for maintenance ).
This seems to rule out failure of the nose gear structure.
I would guess and it is only a guess that a miss-positioned shuttle valve directed hydrulic PX to the retraction line.
Shuttle valves can get mis-positioned during some maintenance tasks such as checking accumulator pre charge PX . the BAe 1-11 could do this very thing if after checking the nose wheel steering accumulator pre charge PX you did not put the nose geer lock in when you put the hydrulic power back on.