There is 4 major factors behind this crash :
1– crypto investors and people made a cash out of profits from the crypto markets for the holidays of the end of the year, this happen every end of the year, that means that a big amount of money is going out from the crypto markets, this is very logical after the huge raise of the prices of the crypto currency, people are greedy, they sell to book their profit this has cause a sell off, what means a huge crash .
2-the rumours of the Hack and the ddos attacks against some crypto platforms such as Yobit and Bitfinex, by some hackers from North Korea .
3-The change in the crypto regulations in some country that adopts Bitcoin earlier this year from Japan, USA, and South Korea,this new regulations will increase the taxes on the Crypto Business World, this means that it will become less profitable .
4-The Hack of the Tether network ( USDT ) the last week .
this crash is very positive for the long term growth of bitcoin and others crypto currency