Might have something to do with the 8/6 PVG-SFO UA 858 flight stuck at Shanghai - it had a lice issue and the Captain wanted to use US pesticides instead of the local supply. Rescheduled as UA 1824 and then cancelled. The 8/7 UA858 was also delayed due to Typhoon. The aircraft for UA838/837 might have been pulled to run the 8/7 PVG-SFO. Just a guess.