Originally Posted by
NWAMileageSlave
You were the beneficiary of a Load Based First Class Upgrade or LBFU.
At the risk of being nit-picky, the term is Load Factor Based Upgrade, or LFBU. They aren't necessarily first class upgrades, as they can be offered from Y to J on 3-class or international flights. Sources:
Upgrades (AA) page on the FlyerGuide wiki and the AAdvantage FT forum thread
LFBU: Non-elite, space-available, counter and kiosk upgrades (consolidated).
I have alerted mods for a potential merge.