Many people are investing in Bitcoin. Therefore, Bitcoins are sold if a person needs money or simply if the current price suits him.
They sell their bitcoin for income and money. We all know that all person needs to have money so that some bitcoin users prefer to sell their bitcoin to gain money. Furthermore, I suggest you must do holding than selling your bitcoin. Just hold it and gain much income.
The reason might be more complicated than this actually. Most of the people are selling their coins because they are afraid that they can lose all their money if they don't cash out today. It is called panic selling which is triggered by media mostly.