"The Value Of Bitcoin Is Volatile In Part Because Just A Few People Own A Vast Majority Of It
Although the recent fall in bitcoin value has been particularly dramatic, the price of the cryptocurrency has been known to vary widely from day to day, and even hour to hour. This volatility is a result of a consolidated market, where just a few people own a huge chunk of all Bitcoin.
Approximately 40 percent of bitcoin is held by about 1,000 users, and the top 100 bitcoin addresses — some of which may belong to the same person — control about one-sixth of all the issued currency, according to Bloomberg. One reason why bitcoin can fall by 20 percent in a day — which is practically unheard of for most equities, and certainly of most currencies — is that if any one of these huge investors sells, it can move the market. Bitcoin's extraordinary price fluctuation is possible because ownership of nominally decentralized technology is, in fact, quite concentrated." -
https://digg.com/2018/bitcoin-slump-explanations