This is economics 101, the price of everything depends on the supply and demand, however while that explains why the price of bitcoin changes and how its price is decided it doesn't explain, at least in an explicit manner, why the price of bitcoin moves so quickly, there are several reasons for that one of the most obvious is that despite the size of this market it is still very small compared to many other markets and as such it is easier to move it compared to them.

Another reason is that there is a great concentration of wealth in a few individuals which we call whales and as such they can move the price with their actions, finally the limited supply of bitcoin plays a huge factor because as soon as the demand skyrockets the supply stays the same and that obviously makes the price of bitcoin to go up very rapidly.