I think in my opinion, it is because last Dec. 2017 it is a long Holiday season and need to prepare a lot of things and it is time giving so maybe most of us are selling their bitcoins these days. Some says it is because some investors are getting interested in altcoins because bitcoin is too expensive to have which is true so that they are trying to invest in good altcoins to gain more profit in the future and I think it can be possible.