To repeat the others, here is one plausible explanation for the "How did this happen?":

At T-15, the gate agent cancelled your reservation and assigned another passenger your seat.

The gate agent may have assigned more seats to other passengers, including giving some seats to standby passengers, and those passengers may have boarded the aircraft. If you were denied boarding, maybe the aircraft was boarded completely full.

As to "how could this happen to a high-revenue passenger?", well, the contract of carriage allows it -- it's linked at http://www.united.com/page/article/0,5046,2671,00.html. Per the summary, "Reservations are subject to cancellation if you are not at the boarding gate and available for boarding at least 15 minutes before scheduled departure for flights within the continental U.S." Even a very expensive first-class fare carries this disclaimer.

Now, you might say "I understand that United wasn't contractually obliged to put me on that airplane, but they nevertheless should have done what it took to get me on that plane, because I'm a high-value customer and will make future purchasing decisions based on this treatment."

Fair enough. Let's suppose for a second the plane was full. UA533 is the last flight of the night on any carrier from the D.C. area to Denver, so somebody was going to stay overnight in D.C. -- either you, or someone who was already on the airplane.

Who would you have de-planed? Maybe an airline employee flying on a "non-revenue standby" basis or maybe some other standby passenger? Or maybe they should solicit volunteers to be denied boarding? Those would have been reasonable things to do, but what if there were no one like that available? Should the agent have bent the rules for you and tried?

Of course if the plane wasn't full, then that's really bad form to leave you overnight. Normally I am sympathetic to the idea that "it's bad form to accommodate a late-arriving passenger because it holds up the plane" -- but that line of thinking doesn't really hold on the last flight of the night.