Almost every altcoin is traded to Bitcoin. Crypto exchanges always create pairs ETH to BTC. Usually traders create the bid or ask orders estimated in BTC coins and such a way they define the balance of demand and supply valued in Bitcoin. That's why the general direction of the global crypto currency market often correlates with the direction of the BTC price movement. Of course, this is a rough estimate.

Sometimes the ETH price is separated from the BTC price. For example, Bitcoin has significantly grown compared to all altcoins this September because large investors were expecting Bitcoin ETF approval by the US SEC.