Maybe because they panic with the price condition that is decreasing. So they sell coins at very low prices. But I think this is a very reasonable condition and when the price of a coin has gone up they will immediately buy more coins. As now the market price is declining and this is the time to reinvest. I see that the price of Bitcoin is increasing every day. So I hope you don't rush to sell coins.
I agree with you they sell it because they see the altcoin market conditions that they hold the lower the price and even most altcoins don't follow the percentage increase in the price of bitcoin and maybe sell it and replace it with bitcoin it is wiser and offers more benefits to get profits