I read this article and now I understand some of the things that are happening in the values of the crypto-currencies.

CME group is offering the possibility to trade with Futures of Bitcoins

CME group is owner of some of the largest and more relevant markets. It owns and operates large derivatives and futures exchanges in Chicago, New York City, and exchange facilities in London. It also owns the Dow Jones stock and financial indexes, and some other markets.

How the futures market operates (from the article)

In a futures market, if the price is $500/BTC, an investor needs to buy 50 futures contracts, each worth $10. If an investor wishes to open a positive position then he goes long with “buy” contracts, and if he decides to open a negative position, he goes short with “sell” contracts. An investor’s position can be either positive or negative for the same instrument.

Consequence

Before middle of December 2017 people were operating in one direction: market going-up.

Since middle of December 2017 there are people operating in both directions: some looking for the market to go-up some others looking to the market to go down.

You can see Bitcoin Futures quotes here.