I think the OP meant all BA-operated flights to/from the US have started using T5B only.
Thence, we have speculation to why that might be....(assuming it is a pre-cursor to something else, and not just an arbitrary logistical challenge BA have set themselves...)
My speculative vote was then than American Airlines might move to join BA in T5B to bring the TATL joint venture under one roof. BA would have to move some more flights over to T3, but would I guess look to prioritize those flights with the fewest premium connections to the US and other T5 flights...