Apparently - according to BA(A) - the problems are attributable to late arrival of staff (baggage handlers?) who had problems with car parking and security screening checks . . .
and:-
. . . was due to take off from T5 for Los Angeles at 1005 GMT. Almost three hours later the plane was still on the runway waiting for passengers' baggage to be loaded.
"Apparently the computer software told the baggage people that the flight had taken off. So everyone in the plane just watched as all their suitcases were taken back into the terminal instead of being loaded on."
(More at:-
http://news.bbc.co.uk/1/hi/uk/7314816.stm )