in my opinion maybe people sell their altcoin to buy Bitcoin, so the price go down, because they are afraid to miss the bull run from Bitcoin, so from that they sell altcoin and buy Bitcoin, but in my opinion not all altcoin are down, because there are several altcoin whose prices have also increased, but the most important thing in my opinion is that you don't panic sell when the price of altcoin you invest go down, because it will make you regret when the price increases again, so my advice is you better hold and buy more if the price decreases.