There are several factors that have caused the crash.

1) Bitconnect going under.

Bitconnect was a website that was heavily advertised throughout social media platforms such as: Facebook, Youtube, and Twitter. This website allowed people to put their money in a "Trading Robot", which supposedly was able to produce returns better than the average daytrader.

This trading robot at the beginning was making returns of "1% daily", which others advertised to their friends. Friends put their money into Bitconnect, under the impression that the bot would make them money, would convince others to join the bot.

Then when Bitconnect had around ~$3B USD worth of capital, their website suddenly shut down. They announced that they were shutting down their lending platform and any balances on the website would be refunded in a currency: "Bitconnect Tokens". Bitconnect Tokens were tied to the value of how the trading bot was doing. But since the trading bot had just been shut down/never existed, Bitconnect tokens effectively had 0 value. People started to realize this, they sold off their share, resulting in the value of Bitconnect Tokens dropping 90% in less than an hour. Many people lost everything they put on the website, causing mass selling of assets.

Worth noting that the "trading robot" never existed, and it was essentially a ponzi scheme.

2) South Korean News Story

A story went viral about South Korea banning trading of Cryptocurrencies. This caused people to sell their assets because they felt that without South Korea's presence in the market, prices would tank so they wanted to be the first ones out the door. This story was false, but by the time it was corrected the damage was done.

3) Influx of New Money

When people saw coins falling in value, they started to cashout. This is because they believed the coins would fall even lower. And with more people panic selling at a loss, this caused the price to drop even further. Therefore, more people panic sold because they believed the price would drop even further, repeat/etc...

Due to the massive growth in crypto over the past couple of months. Many people are entering the market and are new here, so they have not experienced this level of volatility and are not used to the price crashing. When the price went downwards, they had no idea what to do in order to minimize loses, so they just sold their portfolio.

4) Bitcoin Futures

The expiry date of Bitcoin Futures is coming soon, this is a contract that allows people to sell Bitcoin at specified price in the future. So profit booking is expected, causing the price to tank.