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.
That is a frequent thing. When there is a negative news it will create a panic and this is what causes many people to sell the coins they have. The act of selling coins so simultaneously can reduce the level of market confidence and this will immediately impact on price reductions. So don't panic and stay calm, so the market conditions will be controlled.