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.
What you said concerning the Federal prosecutors maybe one of the reason why the market is facing a down trend and we both know that the price of every crypto currencies are manipulated sometimes but crypto market correction usually happen the next year when an ATH has achieved. However, I can assure you that nothing the probe which was to be launched by the federal prosecutors can really to stop market manipulation.