The reason is very simple, maybe just i need money so that i would sell coins to survive, or i got profit from the coin i sell, business is business, i'm not a coin holder.
All those investors who are holding any if the crypto coin have their own reasons to sell those coins. However, one of the most common reason because of which most of the investors plan to sell their coins is to have some money in their wallets which can then help them in meeting all of their needs and wants. Other than that, there are investors who sell because they get panic due to downfall in the market value of coins.