The price of bitcoin is determined by supply and demand of the market. bitcoin is a decentralized currency so there is no central organization that determines its price.
I agree, and the one who create supply and demand are traders. In short, investors and traders are the people who make the bitcoin price either increase or decrease. Aside from that, the hype and FUD also affect bitcoin price. Who ever weigh more between this FUD and HYPE will definitely markets movement.
The only issue I see with the supply/demand argument is that news about Bitcoin, good and bad, dramatically affects the market way beyond what it should be, realistically speaking. Every single piece of news seems to be treated like a major market mover, and it skews the demand for Bitcoin up or down, depending on the news of the day.