The volatility is a result of Bitcoin being decentralized. And since Bitcoin is decentralize, the only thing that dictates the price is the demand for Bitcoin with respect to the total supply of BTC available in the market. The law of demand and supply should suffice for the explanation but I'll try to make it easier. The price varies depending on the supply and demand and since we cannot control the demand for bitcoin, as well as its supply, then if a person bought some Bitcoins then we will see a price change, that is also when someone sells his/her coins to an exchange.
The demand and supply is uncontrollable which then results to volatility.