Just arrived home.

I learned about the extra segment checking flightaware.com and ual.com about an hour before departure, which is when I went to the RCC desk to have them confirm it. The reason stated was "Customer"

I got to the gate about 10 min before boarding and they hadn't announced the extra segment yet. About five minutes before boarding about four or five suits showed up to watch the reaction to the announcement. We were told that a STL flight experienced mechanical problems and had to return, stranding some passengers. The load on our 319 was only about 40 people, so we were going to run down to STL, do a quick turn, and then fly onto LAX, with an hour delay in arrival. There were a lot of groans in the waiting area and UA offered to rebook anyone who wanted to a flight the next day (no VDB, though). There were no takers.

We boarded the flight, but about halfway through, the captain exited the plane. We later came to find out that he decided to refuse the routing to STL and I am sure there were many conversations with dispatch and other impacted areas. It took about 15 minutes for a new flight plan to get created and filed and we arrived maybe 20 minutes late into LAX.

One has to wonder what was done for the stranded passengers at STL, especially with the 11th hour change in plans.