That could very well be the case, but if that were really the cause you'd expect to see this pattern every year. However, it apparently didn't happen for at least two years in a row[1]. Also, I would have expected Skype engineers to have wised up to the situation by now if it were really a seasonal thing.

[1] Since the last outage was 3 years ago. I can't find a source for that right now -- would be interesting to see if the outage was also around Christmas time.