I think the most helpful answer is that this kind of thing is as common on UA as it is on any other carrier. To wit, there are unexplained instances of FT posters having seat-related shenanigans happen to them on every board, but in years of flying (mostly UA, now DL) this has never happened to me, excluding obvious things like change of equipment (which, as an FTer, I was usually expecting). It sounds like your first one was a cancellation though? Flights get canceled; you're lucky if you've never had that happen before.