My booking had been placed on an SAU — Space Available Upgrade — list. SAUs can be added manually by the airline or are available to AAdvantage passengers who exchange miles in return for being added to the list. They only apply to the individual, however, so travel companions aren't eligible.
Being on the list means that once you're at the gate if there are empty seats going in the cabin class above you, you might have a chance of being offered an upgrade — but it's not guaranteed. I got lucky and was surprised to be told at the gate that my assigned economy seat had been swapped for one in business class.