I think it's because of the good and bad news about Bitcoin. News is one of the factors that influence the price movement significantly. Bitcoin users who hear negative news about Bitcoin, will feel panic and eventually sell their Bitcoin to secure the assets. As a result, Bitcoin's price will drop due to large-scale sales of Bitcoin. Vice versa, if there is positive news about Bitcoin then many users who make purchases. Of course with the purchase of Bitcoin on a large scale impacted drastic price increases.