First thing they will teach you in Economics class is the Supply and Demand. And this mainly affects the price of Bitcoin. What is amazing about it is that it is being 'controlled' by the consumers unlike the other currencies which is being controlled by the government. Yes it is volatile and a lot of other factors may affect the price. However, what's good about it is that it is universal and no country has the power to manipulate it's price.
Because not all bitcoin holders are the same financial status, there are some investors that can afford a big amount to put as an investment in bitcoin and the most powerful are those who had a lot of money that they can even do play the market in investing a lot of bitcoin and then waiting to sell when the price goes up.
We can avoid this thing, there are always who want to control even in different aspects of investment. I will not surprise if bitcoin also has, a fixed amount that easy for them to control, why not. The demand and supply will always vary on the price movement.