What's Causing Jan. 4th Crowd Spike?

I’m looking at going to Disney in Jan. with Jan. 4th being our first day. According to Touringplans, for the past several years, Jan. 4 seems to have a spike in the crowds, and registers as a 10 on the scale. But, if you track the WDW marathon, that definitely has not always impacted crowds. In fact, more than 3 years ago, it didn’t have ANY impact on crowds. Also, the end date of the marathon seems to have no impact on crowds. So, I’m thinking its not the marathon that is impacting this giant spike on Jan 4th.
If its not the marathon, what could it be? Any ideas?