It might just be that there are more flights scheduled in at that particular time then stands available on the main terminal.
I really don't profess to understand how, why, when and where which flight gets what but there are often flights that will almost always be on a remote stand. Possibly if the aircraft rotation is from international to domestic.
In that case the inbound aircraft (yours) coming from an international flight cannot park on the domestic stands thus you need to be remote and the domestic passengers 'bussed' to your aircraft after you have disembarked. These regulations are not of BA's making they are from the Department for Transport.
It is often not a simple and clear cut as many passengers assume. When I was on Short Haul we, as flight crew, were banned from getting off the jet during a turnaround and going upstairs to departures for a cup of coffee without clearing security ...... twice.
p.s. Watch out for the LH rotation through MUC in the winter. The aircraft is often delayed at Frankfurt with de-icing. Just a heads up.