The price of bitcoin is formed exclusively by supply and demand for it. Bitcoins are not secured by any tangible assets, but the same situation is with any Fiat currencies, rubles or dollars.
Bitcoins price is influenced largely by demand and supply, and demand and supply is influenced by all other factors associated with bitcoins. So basically, when the network is running smoothly people build faith in the technology and are are willing to buy, but when there is a chink in the chain, panic reigns.
And also this is nothing like fiat currencies. There is a delegated sector which supports and regulates the worth of national currencies