There is an option in Amadeus Altea to link multiple bookings - both in the case of single passengers connecting across two bookings, and linking multiple passengers on equal itineraries. Some airlines allow you to include another passenger directly in OLCI. In this case I suppose some link was added while TCPing, which generally is not done.

Never seen this with BA, but seen this with a number of other airlines so I'm not too surprised - the part that surprises me is that the linking was done somehow behind the scenes though. Unless this is the return leg and you all checked in together at the airport for the outbound?