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Depends on the demand and supply. It's the buy and sell pressure that sets the bitcoins price. The more buy pressure (high demand), the higher the price will go since people will be selling their coins at higher price as more people are willing to buy. When the sell pressure (high supply) increases, the exact opposite happens. People are willing to sell their coins cheap for whatever reason and buyers are setting price even lower, causing the price to drop. The buy and sell pressure eventually reaches an equilibrium point, and that's the less volatile price we see.
^ Definitely right, the basic principle is because of the demand and supply and the price will calculate based on the buying pressure increase and it will drop when selling pressure will increase. It was determined by the last trade on a specific exchange which is what they called the last bid. That is the reason becomes BTC price is very volatile because there are many people who buy and sell, and it depends on what they heard on the news, when there is a FUD, eventually the price goes down. And some are manipulating the market.