December is a holiday month and in January people usually don't have much money left and they don't make big purchases or investment decisions, so February seems like a good time to conduct a sale. Especially if they expect the market gonna go up.
Last bullrun was in December 2017 and I think next will be too. And its not depends of holidays. I think it depends of positive news and we have some big (Bakkt launch on 12th December)