Long story:
Crypto / BTC prices have fallen from their December rates due to:

1 - Bitconnect pyramid collapsed;
2 - Japanese exchange Coincheck was hacked few days ago, for a total of 530 million USD missing;

3 - Tether (USDT) token appears to be a pyramid that “printed” (or should I say, minted) a few hundred millions Tethers to cause a surge in the need more Bitcoin to be bought (because Tether needs to have a constant price of 1 USD, 1 USDT = 1 USD) and more Tether to be bought with these Bitcoins. In the end this leads to Bitcoin price increase. It is said that the exchange Bitfinex is behind all of this. Bitfinex created USDT (Tether).

EDIT 31st of January: 4 - According to an article from this morning on The Telegraph, Coincheck hackers are moving the stolen coins to the exchanges and are trying to sell them:
Coincheck hackers trying to move stolen cryptocurrency after major Japanese heist

I highly suggest you to read the Medium articles on the Tether topic by the guy who brought the whole scheme to light.