According to CNBC, federal prosecutors had launched a criminal probe into the cryptocurrencies earlier this year based on suspicions that their price was being manipulated. I think it might be the main reason behind the price of bitcoin dropping to around $4000, but I also wonder whether it could be that authorities are afraid that Bitcoin and other cryptocurrencies are taking over the financial system since they're now so popular.
The cryptocurrency market is not so big as to pose a threat to the global financial system. As for the manipulations, they are. But if we look at the situation in the world as a whole, we will see that the markets are falling. Therefore, the fall of cryptocurrency is predictable.