Many altcoins move in similar direction to BTC, if BTC experiences a DIP other altcoins also experience a DIP, and when when it goes up the follow similar directions, recently many altcoins have experienced a DIP in what has been a very tough year for them , the altcoins are already experiencing more DIP recently ,
And the delaying of the US SEC on the approval of a the ETF will also affect the DIP of altcoins.
Investors of altcoins are already selling it, BTc is able to maintain its value.