It happens with simple logic. The price of Ethereum is directly positively correlated with Erc-20 tokens, for purchasing any token traders need to go buy the first ETH. There is the reason behind this direct correlation but exceptions are not rare in the crypto market. New investors visit the crypto markets because a big part of them just heard about the Bitcoin. This correlation index goes down if Bitcoin dominance and price goes up together form my understanding.
ERC20 tokens and Ethereum prices have close correlations, nearly the same as the correlations between bitcoin and altcoins. Things become clearer now with this comparison, right? Ethereum goes up, ERC20 tokens go down, and vice versa.