Probably a situation where the two FA's were positive space and the uninformed fool of an agent saw QUAUP fares and decided to make up her own little rules.

Downgrading two paid A fares to E- without explanation should be a fireable offense. I would have told her to offload me, refund my fare, and immediately demanded to see the station supervisor.

Please write to UA and tell them what happened. You are not only due a refund but also a personal apology. Don't let bad apples get away with this kind of thing.