Thanks for the extra detail, starspook. That really does help!

So it was almost certainly caused by the change in your outbound. Basically, the AA reroute in the AA system caused the loss of the reservation for the inbound BA in BA's system.

Note (as discussed above) this wasn't the loss of 'seats' - it was the loss of your actual reservation on the BA flight.

It seems like more of an AA than BA issue to me, since AA were the ones responsible for the ticket - that's who I'd be complaining to. However others here may have a better idea about whether it's worth chasing BA for anything.