For anyone NOT wanting to read a novel: the initial DL287 was delayed a few times, then ultimately canceled due to crew time-out.
Replacement flight DL9891 took off the next day, had some issues with Russian airspace, and then returned because everyone's GREEN CODE had been rejected by the Chinese govt.
Don't know why though, and lost interest in reading this overly verbose drivel from a disgruntled passenger...


Added: UA857 is still flying (and flew over Christmas, and has a plane in the air SFO-ICN right now) with a route of SFO-ICN-PVG, so I don't think it's related to crew/Seoul/disembarking, etc.