Nobody and everybody. It's the basic principle of supply and demand. If the demand outweighs the supply, the price will go up. If there is less demand, the price will go down. Bitcoin is still relatively young and very unstable as people tend to easily panic an fall in the manipulative games of whales.
The volatility however has been improving over time, and compared to a few years ago it is: 1) Less prone to major percentage crashes. 2) Less historically volatile
1.
[1] You can track this data using the following website:
https://bitvol.info/.