I won't make excuses for BA (I actively avoid flying with them anyway for a variety of reasons) but surely the delay in telling people that the baggage had been left behind was an unfortunate side effect of the IT breakdown?
If the system is not working, it cannot tell anyone that anything is wrong.....so bags that were checked in have not been through the system due to the system going fuzzy, so that system cannot alert anyone that bags are stuck where they should not be (if that makes sense)
One bag from one flight goes walkies and it is accepted as something that happens occasionally
All bags from all flights do not make it out onto the apron to be loaded due to a full IT tantrum, then getting the message out to all planes, all pax and the destination airports is going to be harder thanks to it being an IT driven system
All bags are scanned by the IT system from the point of check in to aircraft and from aircraft to destination carousel, if one part of that IT journey is not working, then that information isn't going to get to the destination is it