Not one, but there are many factors that affect the prices of these volatile crypto coins. Moreover, the FUD that media creates in the minds of the people., creates panic throughout the community and leads to a disturbed atmosphere. Such kind of panic is completely disgusting for everyone else in the community. So now your question what causes the fall in the price of the bitcoin...the exact reason cannot be predicted, because price changes is the result of various market factors affecting the BTC price, so you cannot just fix responsibilty on one stupid reason!