We can't really assume that this will always be the case, remember that alt users have the word "alt season" for a reason. By my experience in observing the crypto market there are times where I see altcoins are down and only Bitcoin is up in these occasion investors have sold their position in their alt in order to ride their capital in the pumping of Bitcoin and there are also times where only Bitcoin is down and altcoins are up so you can't really put that in stone where there is always a correlation involve, there is no guarantee that alts will always move the same way as Bitcoin.