BTC Rally is followed directly by eth. Even if you don't move the price of other altcoin, eth will move. There is a specific price difference between Bitcoin and ETH. If the price of Bitcoin increases more and the price of Eth decreases, then it will be equal between the specified period.
If the price of Ethereum has decreased but Bitcoin has increased, then the price of Ethereum has decreased for only a moment. Going back up in my opinion. but if Bitcoin goes down a lot then everything will return to its old price.