There is always some direct and indirect correlation between the two. In the last year there was completely different scenario of the bitcoin and altcoins. In that case when BTC used to go upward then ALT used to come down. Because people had feeling that this is the time when BTC wil profit more and thus they immediately used to sell the ALT and trade it to the BTC. Thus BTC used to spike even further. Vice versa was also true about this pair.
However after the 2018 things have changed, they have got directly proportional to each other and whenever there is downfall of the BTC then there is downfall of the ALT and vice versa. Why it is so, its difficult to say.