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.
I don't think the financial system fears cryptocurrencies that much as no decentralised coin has been favoured or backed up by any central organisation. I think its more to do with the fear amongst the holders and liquidating their investments.