Was on BA54 JNB-LHR yesterday (14/12) on G-BYGE. Flight was delayed about 2 hours and left from remote stand D32 at JNB. We'd taxied almost to the runway when the Captain announced that we would have to return to the gate to have the cargo re-arranged as the aircraft was out of balance.
This took a further 45 minutes or so.

I'm curious as to how this would become apparent during taxi but not be apparent before the aircraft left the stand. I've never experiences this before. On arrival at LHR - after a 25 minute wait to get on stand - we had an additional passport check at the top of the airbridge. Never experienced that before either.