Recently, a group of cryptocurrency researchers and traders suggested that the bitcoin price has began to fall from its all-time high on the same day the bitcoin futures market of CME and CBOE launched. They claimed that through the futures market, institutional investors and large-scale retail traders manipulated the market to cash out short contracts by purchasing and selling massive amounts of bitcoin in a correlated manner: Bitcoin Price Drop From $20,000 Likely Due to Market Manipulation
With cryptocurrency, people are often very unsure why it has any value at all and are afraid that at any moment it may become worth nothing. This creates a situation where if the price of a coin drops suddenly, many owners will try and sell as soon as possible, before the price drops too low.
To sell quickly, you have to offer a noticeably lower price than the other sellers. This business of undercutting the other sellers causes the price to drop very quickly, and this continuing sharp drop makes even more people afraid and want to sell.
But, in general, large part of the correction could be natural. Bitcoin experienced an extreme spike in value that year and the correction keep the value of the coin so high above its long-term average makes it look like Bitcoin has recognised value, but not as high as the speculators in late 2017 had hoped: Bitcoin Traders: Bitcoin Price Drop From $20,000 Likely Due to Market Manipulation -- How?