People sell bitcoins because they are afraid of the volatility of the bitcoin's market because they can easily make and lose money and if they think that they earned enough money for their expenses then they will just sell bitcoin without thinking about it for twice and that is the typical bitcoin user but if you are really into bitcoin then you will hold it for long term.
In my opinion Bitcoin is made for the purpose to buy and sell it. Now there are various reasons of selling BTC, many people selling means many reasons at a time. Everyone has their own reasons to sell in which I think the most recognized one is that people can hold till the time they dont need fiat. At any time anyone needs fiat they immediately sell the Bitcoin to collect fiat. From a trader point of view the selling of Bitcoin reason will be different. The trader may sell Bitcoin to take profit from high price. Some of the people sell Bitcoin due to a panic situation in the market.