I think bitcoin and altcoins are one dependent stock ... if bitcoin rises altcoins will go down ... whereas if altcoin rises bitcoin will go down ... that's one that proves that bitcoin and altcoin are dependent or related ... thank you ....
Although it tends to be like that, it's not always like that. There's just no correlation between the two. If bitcoin goes down in price, I'm not going to buy ETH or any other altcoin. I'm simply going to wait for the price of bitcoin to go up once again. There are times when all the major coins and bitcoin are all in negative gains. It's because people are cashing out their cryptocurrencies.
I also observed what you are saying when the price of bitcoin was moving from 1K$ to 2.9k$. You would literally watch a show of altcoins going down. It's because people want to cash out their money through bitcoins. But as time goes by, I also observed that it's not always the case. It's only the case when bitcoin's price is significantly moving.