So had never experienced this issue previously with UA, but had my first random seat change was on one of my flights on Tuesday. I had received an "elite" upgrade five days before the flight and was assigned seat 3A on an ERJ175 which was fine with me. I go to do OLCI at 24 hours to find I was now in seat 1C which was not good as I knew I may need the seat in front of me to store my luggage so changed my seat to 3D. It was all rather strange why they moved my seat,. I was not complaining as I had not paid for the seat and it was a short LAX-SAN flight, but did find it curious that they moved me and wondered if I should have "confirmed" the seat on my app as perhaps this would have kept the seating.
(Although not sure it would have made any difference to the FA as she kept referring to the person in seat in 2D by my name and the passenger in 2D looked very confused why she kept being referred to as "Ms Aussienarelle" so am guessing the manifest was completely screwed up for the seating arrangements, or the FA cannot read...which may have been a real possibility as she moved the luggage for the passenger in 1A to 7D while he was in the lavatory and we needed to tell him when we arrived in SAN where she had moved the luggage.)
Again, it was a CPU so not complaining but all in all the change in the seating assignment and the FA was an interesting short flight.