probably because the price of bitcoin and ethereum goes down so people sell their altcoin to switch to bitcoin or ethereum, and that's why altcoin prices go down, and bitcoin and ethereum prices go down because those who sell them panic when they see bitcoin and ethereum go down a bit and sell it straight away for fear it will not rise again (according to my prediction).
I agree with your opinion this is all happening because people start panic with bitcoin price down so many people sell it, so that's the bitcoin price down