Bitcoin's value is tied to its demand. If the demand goes up, so will its value. The main reason as to why bitcoin is not so 'stable' is that it is entirely decentralised (mostly). The whole network is built on a platform of user trust. The only reason why fiat is 'stable' in its own way is that it is backed up by the government, however that same government is kind of screwing it over by inflating it in a bubble.

Bitcoin's price was stable before August 1 and for a few days after. Then it shot up like crazy, which it what we can see now.