First of all, Christmas is the same every year, but I don't think bitcoin is falling in December each year. It did last year, sort of, and we can say that it does this year, but what about, say, 2017?
As for personal usage, I did sell some BTC to buy presents for New Year and I also received my New Year present in Bitcoin in advance. So some movement can be explained by actions like that, but I don't think it's likely that this is the main reason the price if going down.
Moreover, this negative trend can be over tomorrow or in a couple of days easily, that's just how volatile bitcoin can be.