When BTC goes up and downs alts are bleeding,while btc stabilize the alts are pumping thats my own observation,thats the reason why the BTC is a killing machine for alts.
I think when Bitcoin price is increasing Altcoins are affected because people choose to dump their Altcoins and shift the investment into Bitcoin. That's why Ethereum network is congested but Bitcoin's network is also the same with Ethereum. If there's heavy traffic for transactions then the network will slow down.