Consider the altcoins, they dont have any literal use case of their own in most cases and they all have been funded by bitcoin or ethereum maximally. They dont solve any problem that bitcoin does and neither have they got any sort of mainstream attention by doing something new that bitcoin has not done. There is a lot of market sentiment associated with this.

Hence it becomes obvious that whenever bitcoin starts a movements, the altcoins will follow it. Opposite is not true, unless it is an altcoin that is well known, like litecoin.

When trading place your buy orders on an altcoin that you might want to buy when bitcoin is dropping, because the altcoin will follow similar trends.