My guess would be a failure of something vital to passenger safety but that wouldn't prevent crew being on board.

I'm thinking of a safety system such as O2 system failure in the passenger cabins rather than comfort (e.g. no working IFE) but as JD Diver says something that is better sorted in TUL or there is no capacity in UK / Europe to deal with it at present - AA will want the plane back in passenger ops asap even if it costs more.