Many people are investing in Bitcoin. Therefore, Bitcoins are sold if a person needs money or simply if the current price suits him.
i Agree with that. Also if there a emergency like medical or for outing they selk it for personal use.
They sell bitcoin because they see that they already have profit on it and start other investment. Some are selling bitcoin because they need money for important things that even there is dump they have to take the risk for it. It is good to save bitcoin and make it profitable instead of spending it.