There are many reasons for cryptocurrency Crashed.
Gambling
Gambling is involved in Bitcoin. Billionaire invest their Billion Dollars, then the price of Bitcoin increased and they Bet for Billion Dollars That the price Decreased and sale their Bitcoin in a very short period of time. Selling of Billion Dollars at once rapidly crashed the bitcoin. Gamblers win Billions of Billions Dollars on every bet.
Hacking of Exchange.
if one of the Bitcoin Exchange Hacked or not responding the price of Bitcoin Decreased. Recently South Korean Exchange was hacked and coin market crashed rapidly.
Cryptocurrency Market
if Bitcoins price Falls down every coin in digital market decreased, because every currency convert in Bitcoin when Satoshi Decreased the coin market cap falls down.