"Why they weren't just delayed departing from Hong Kong is uncertain."
They weren't delayed leaving HK because they weren't leaving HK. They were landing there, but were unable to because of the weather. What's worse than getting stuck on the tarmac for 16 hours at a random mainland Chinese airport shortly ater takeoff? Getting stuck on the tarmac for 16 hours at a random mainland Chinese airport after having been in the air for the previous 16. That really sucks.