What was the plane type? The fact you were boarded and then removed makes me think there was a Wright restriction. There may be an entirely set of rules that govern that, and the "United ticket" being less valuable MAY be, based on whatever method AC uses to deplane on a capacity reduction, their method of prioritizing the reverse order of removal.
Did they replace you in your seat with another passenger, or did the plane leave under the seating capacity due to a weight restriction?