All second-hand from people at the airport ( but this is a rumour site )
1. BIA was closest diversion location with United presence
2. Passenger & baggage removed into gleeful hands of PSNI swarm
3. Aircraft fueled for departure but problem encountered securing hold door
4. Given that flight was already running four hours late, crew timed-out whilst trying to resolve door problem
5. Crew decamped to hotel and passengers played find-a-room lottery
6. Crew have exceeded their duty hours for the week and new crew being flown-in from Heathrow
7. Departure anticipated Sunday evening
The sequence probably won't surprise anyone who has encountered disruptions on United. They never, ever, ever pick up the phone to Omni / Titan / Euroatlantic for a contingency substitution.
Edit: Belfast has handled 777s before, but very infrequently. Last I recall was a TNT 200F in 2013.