I'm quite sure it's an aircraft change, and the fact that it's HKG-TPE or v.v. makes it even more likely. I have this happen to me quite often before, esp e.g. first flight HKG-TPE ~8:30 am, pretty much changing every day or so. and yes, the IT system might automatically assign you a similar seat, but this is not immediate at all - very often it would take a few hours, so I'm actually not sure if it's automatic or manual...