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.
Panic selling is important but it is not what makes the price of bitcoin to go down in a sustained way, people are not buying bitcoin, it does not get more simple than that, people are seeing there is not a huge potential for profits and they are deciding to leave the market and are not going to return until that changes or they have such huge losses that they are leaving the market because they were not profitable.