From my knowledge of escalators this is only possible if a member of staff had actively inserted their key at either the top or the bottom landing and reversed the direction, i.e. it wouldn't have just happened spontaneously nor could a customer have done it.
...and if procedure were properly followed, this certainly wouldn't have happened with people riding on it at the time... a member of staff should approach from the bottom landing (as it's going up), unlock and pop out barriers, then ride the escalator to the top, check it's unoccupied, then insert a key and stop, insert into a different barrel and change direction, insert back into the original barrel and re-start, then ride back down to the bottom landing and stow the barriers back away.
I'd suggest letting LU customer service centre know just in case there is a fault local management should be aware of...