Many people are investing in Bitcoin. Therefore, Bitcoins are sold if a person needs money or simply if the current price suits him.
Some people sell their bitcoin to have money otherwise they sell it maybe because the value of bitcoin increases. But unfotunately maybe they were not comfortable in saving for investments because they were afraid to be scammed or they don't believe that they really can earn profit in bitcoin.
If the price rises, people usually expect the maximum increase in the price. That is, as soon as it starts to fall in price, it is immediately sold.