I highly doubt they bumped someone from the earlier flight to accommodate you, that would have been very wrong and is simply not done. It's more likely that they had a bunch of people not check in, so the flight was no longer oversold closer to boarding time.

Perhaps the new standby system which ranks standbys according to status kicked in, or the agents ranked you ahead of the non-status standbys manually. If the other two customers had lower status, or no status, or were sent over from AA, then you should have cleared ahead of them anyway and I don't see what there is to be shocked about.