I've been u/g a few time without request or expense as a 2P/1P on a discounted ticket due to space reasons (say there is a family of 3 that want to sit together, and you're in that 3rd seat they'd like to have, so they move you up to Y/B). This is most commonly done during peak travel times (holidays/vacations) when people travel in groups.

Sometimes, depending on the airport/counter/gate/crew, I've been u/g just out of them being nice. This usually happens at BWI and IAD, where they crews are familiar with me.

Just enjoy it.