Hi,

Possibly, it was a bag belonging to a passenger who for some reason failed to board ( ie got stuck at transit security ( or for domestic flights being delayed at immigration) or failed to get to the gate on time.
With containerisation , I think it is possible that all the bags are containerised and sent to the aircraft before the pax board so by the time boarding is complete the luggage container could be on board so if the pax fails to board the container has to be removed to get the bag.

It is relatively rare ( but not unusual)- i have had several on the domestic legs LHR-EDI.

However it has usually resulted in a delay of around 10-15mins ( and possibly slightly more for a new slot) and not 1hr.

Regards

TBS