usually when bitcoin is rising, altcoins drop.
again: market cap = price * supply when price is going down their market cap goes down and since there are 1700+ altcoins most of which have billions of coins the result drops harder.
now on top of that we have the altcoin bubbles that are still with us thanks to last year's market wide altcoin pump which means they still need to get dumped harder to reach bottom.
for example you have 100,000 XRP and go on Binance and sell it for bitcoin. you get 6.13BTC. now if anything you now own bitcoin which you may sell for USD and crash bitcoin price. in other words the exact opposite of what you said is true meaning when someone sells his altcoins for bitcoin it is possible that bitcoin price is pulled down because they may sell some of the bitcoin they get for USD and crash the market.