There are many reason for volatility of Bitcoin and some other cryptocurrencies.
Mainly because the total supply is not enough. If any institute or individual is powerful enough, then they can influence the market price
relatively easily.( Just imagine if there are only 100 apples in the whole market, and someone is rich enough to buy 20 of 100, then he or she can influence  the market price by buying high and selling low)
The another reason is cyrptocurrencies  don't like the traditional fiat currencies or securities.
There are no back-up and the market price is fully based on transaction.