Metallo:

Welcome to flyertalk. You were the beneficiary of a Load Based First Class Upgrade or LBFU. These upgrades are offered via the check-in kiosk when the revenue management people predict that there will be unsold first class seats after all elites who want to be upgraded to first class have been upgraded and that everybody who wants to purchase a first class seat has purchased a first class seat.

How often will you see one? Depends on when and where you travel. If you stay away from elite heavy routes (i.e. LAX-JFK) and elite heavy travel days (i.e. Monday or Friday) you might see one again.

Rick