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.
Indeed, many reason why investor selling their altcoin. But mostly, they selling their altcoin because the project looks dead and the developers team not active anymore. Beside that, investor prefer holding bitcoin rather than altcoin because bitcoin is much safer investment