Did you leave your watch on the charging port? I am not sure if this is the cause. Because I notice it is kind of different compared to Phone, if Phone has more than 50% or if we want to take the risk not not plugging into the Charger, the phone wills till perform upgrade. While the watch seems need to leave it on the Charger from upgrade start till end.
If not help, I would suggest try contacting Apple support.