I've had at least two times when I've been upgraded on award tickets.

First time was a looong time ago, definitely pre-merger UA, and before there was ever such a benefit as being able to CPU due to holding a CC (and I've never had a UA card that has that, anyway). It was ORD-CVG. I think I was gold, and upgraded at/close to the window. This was a YYZ-ORD-CVG itinerary. Traveling just me. Have no idea what happened there, but Y was not even close to full.

Second time was traveling with my family...I think we were 3 award tickets and 1 lap infant, and IIRC, were gold at the time. CVG-SFO. This was the holidays, and I don't think we showed up on list after checking in (as expected). But I remember getting a message as we were walking from check in desk to the gate, and wondering how this happened. These were definitely the last F seats, and Y was oversold. So I suspect that was the reason