I have flown extensively for years, but for the 1st time ever on Sunday, I had a flight cancelled.

We were returning from LAX to DEN on UAL. The plane which we were supposed to leave on was incoming successfully and on time from SFO, so it appeared that we were good to go.

However, about 45 minutes before we were to board, UAL reallocated our incoming jet to carry a load of passengers from LAX to SFO. Same gate, same departure time and everything, UAL simply notified the passengers outbound to SFO to report to that gate instead of the one where they were at. So our flight to DEN was cancelled and "our" plane rerouted to SFO instead. I don't know what happened to the original plane that was supposed to fly to SFO, and UAL wouldn't address why our flight was cancelled so that the plane could be used for another purpose.

We were able to reschedule and get on an outbound flight to DEN 4 hours later on UAL, but many people were on standby and unable to fly out that day. United ended up paying some people compensation for having to switch to a Frontier flight that evening as well.

Does anyone know how these decisions are made? Obviously, the flight to SFO pulled weight over the flight to DEN for some reason. UAL would not go into why or how the decision was made.

It doesn't seem like it would really improve the situation of being one plane short because they still had to reschedule a large number of angry people to DEN. Unless there was a passenger on the SFO-bound flight who was influential enough to compel UAL to rescuedule in this manner, it just seems like it would have been easy enough to let the SFO flight that didn't have a plane cancel instead to cancelling the DEN flight instead.

Any thoughts?

Well, I can come up with a few possibilities:

1) They needed the aircraft more in SFO than in DEN (maybe DEN had spares that could replace the canceled flights aircraft down the line). For example (totally made up), say the lines of flying after the LAX flight were SFO-SEA-DFW and DEN-BOS-IAD. If SFO has no spare aircraft, UA would cancel a total of three flights (LAX-SFO, SFO-SEA, SEA-DFW) but would only cancel one flight by canceling the LAX-DEN flight (and using a spare from DEN for the remaining two flights).

2) There was little to no availability from LAX-SFO the rest of the day with better availability to DEN. Then at least they rebook all the DEN passengers for the same day rather than putting people in hotels over night.

3) LAX-SFO had more connecting passengers who would have had screwed up connections (possibly to international flights). So it again was easier to rebook the DEN passengers who were just headed to DEN.

These sound like reasonable reasons to me. As the OP said, UA was able to move some pax over to Frontier, maybe that kind of option didn't exist to SFO.

You should consider yourself very lucky then. I have had a few cancellations (and many delays) where a 4 hour delay would have been a great improvement over the actual outcome.

I'll go with #3 of croberts134's list.

The first time I flew to India I was taking CO from MCI to EWR, then the nonstop from EWR to DEL. The first snowstorm of the year blew into the Midwest and everything got snarled. I had a 3-hour layover in EWR and my flight out of MCI was posted 3.5 hours late. I was on the phone with the travel agent sorting out options when my husband came over and reported that we were now posted "only" 1.5 hours late. Phew. I suspect that they figured out there were a lot of people trying to make international connections in EWR (where the weather was just fine) so they grabbed a plane meant for another destination and put us on it. I was very grateful to those displaced pax, if it makes you feel any better.

I also felt sorry for anyone who decided to leave the airport and have dinner since their flight was delayed 3.5 hours.

When I worked in ops, there were an almost infinite number of decision points.

It could have been that the crew didn't have the duty time left to fly to DEN. The fact that Frontier has a flight that they can book on definitely plays a part in deciding which flight to cancel, but I would assume with LAX-SFO being one of the busiest lanes in the country, it would have been the other way around.

Also keep in mind that almost everyone flying the LAX-SFO leg were either O&D or international connecting passengers. DEN is largely a domestic hub, which has far lower yields. Of course that's all speculation anyway.