Dear, have you heard about Segwit ?

If yes then that is the reason and answer for your question.

If no then let me tell you, Segwit is an update planned to ease the transaction of bitcoin and is going to be implemented on 1st of August. Now the people running the transactions called miners disagree with each other over Segwit issue. Some want it to be implemented and few are not interested in Segwit and demand a more effective solution.
It is believed that if difference between these two groups of miners is not resolved then we may see bitcoin split into two different currencies. If this happens then bitcoin will loose it worth to a great extent. Now people are selling bitcoin and trying to avoid the effects of Segwit clash. This has resulted in crash as large number of people are cashing out. This may continue upto August 1st and then we can predict new future of cryptocurrencies.

I hope i answered to your satisfaction, stay calm n keep watching the market.