Panic selling is what draws the price of Bitcoin so down to the drain. Any event which leads to panic selling of Bitcoin can be considered as a contributing factor. The news of a hacked exchange site, false news from governments, banks and the media, a Bitcoin advocate country later banning Bitcoins etc. All these factors contribute to the fall in price of Bitcoin on the market.
While panic selling is without a doubt a contributing factor, most people do not begin to sell their bitcoin as soon as the price goes down, panic selling is the consequence of a huge dip in the market, then people get scared and finally they want to get out of the market regardless of the consequences.