It's my belief that problems like Scott's often don't have one single cause. I don't question that he is affected by SAD--I have a touch of it, too, as I think I've already discussed on this thread.
But if, in addition, he doesn't have good memories of past holidays (I thank God I do), I can well imagine this time of year, as the days grow shorter and colder, it could feel like getting hit with a double whammy.