Well ideally bitcoin is a means of investment, but if you really need the money for whatever reason, you have to sell the coins to get some sort of fiat currency back.
True, having a bitcoin is a form of investment but when time comes that we really need a cash with us, we have to sell our bitcoin. But if your are rich and doesn't need a lot of money or you have an extra cash with you, bitcoin can give us a good investment if we hold it for a long time.
In my opinion there are two main reason for selling. One of them which is quite happening is panic selling as most of you know. When bitcoin holder sees that prices are going down sharply, he or she get scare and start selling with the herd. Second reason is the need for cash.