In fact if btc is pumped the reason other coins are dumped is because everyone is leaning towards investing in btc due to which the price of bitcoin increases and the price of other currencies decreases. These usually increase by increasing the demand of the market not surprisingly the price of bitcoin as well as other altcoins will go up bitcoin will be at the top of all if bitcoin performs well the price of altcoin will increase.
Usually though when the price for bitcoins move up then almost every crypto moves up some might be slow and some faster but the usual trend obver the years have been like that. Now it is a bit different that the price in alts is going down because I would understand if the price remains stable for alts but decreasing price might show some serious surge in investors towards bitcoins and because of so many bad coins maybe investors are now losing faith in the altcoin market.
the shit coin investors leaving for btc, simple as that
You may call any coin as shitcoin but BNB is a solid one man and it allows to reduce fees on Binance exchange so it can't be categorized as shit and Ethereum has been the backbone in the crypto space.