Bitcoin is volatile because there is a limited supply only and it is just 21 million bitcoins. It is also difficult to mine bitcoins because as far as I know it is divided into two after 4 consecutive years. I think that is the reason behind its volatility. Its price goes up when there is a great demand but the price becomes lower if the demand will goes down. It is a matter of scarce resources.
As my opinion bitcoin is volatile because so many reason one most reason of this is the demand and supply of bitcoin in the market. If more investor selling their bitcoin in the market then we expect the price diclined in a very low price. And if the their is shortage of bitcoin supply in the market then the price will inclined in a very high price. So bitcoin price is like a pendulums in every actions their is an equal reactions.