It's been in place for about 15 months now and was part of a wider work between AA, BA and Heathrow to improve the transfer process for customers connecting onto AA flights. The security requirement for AS carriers and the large volumes transferring to BA to AA meant that the T3 flight connections area was constantly congested and customers were missing flights. As CWS said, the wait for the transfer bus presented an opportunity to get this part of the process done earlier and utilise the wait time. Once customers reach T3, they now just need to clear security and not make another stop as they did before.