It is difficult to pin down any factors with any reasonable certainty.
Here’s my best guess:
Bitcoin and Ethereum crash every January because of a combination of profit taking, Christmas/New Years spending. I also assume a lot of Chinese investors that got in are selling in anticipation for Chinese New Year. This is significant because people literally give out cash in hongbao, so they need cash.
In particular, this January there is a lot of FUD news on South Korean bans, despite clear intent of 10 new exchanges (including OKCoin and Huobi) being opened there, and a lot of new investors see a drop, get anxious and sell off.
It was clear that a South Korean politician was creating false and misleading statements to profit on a drop in price. However, this wasn’t widely reported on the news and people are often more 2x reactive to fear. In fact, it was finally reported recently that South Korea is simply making trading on exchanges legalized, with taxes and regular KYC/AML to prevent anonymous trading - which is pretty much like every other country out there, including crypto friendly Japan and the US. Unfortunately, a lot of Bitcoin bears want to take advantage of fear and keep spreading fake news with negative sentiments.