This shines a light on what is, most likely, a policy that is not in the interest of passengers.

In my experience, the airlines offer a decent incentive for taking a bump and they do this before boarding. I know delays have been crazy the past few days, so maybe people who were already delayed didn't want to wait any longer?

I wonder what kind of incentives were offered? You'd hope United would offer enough to get someone to volunteer, especially in cases like this that are so late into the boarding process. Why not just get a United official on the plane to start raising incentives until the $ amount makes someone bite.

Terms and conditions of the contract may be legal but it could very well result in incidents like this.