There are different situations, usually the most popular cryptocurrencies grow together. But it also happens that one grows and the other falls. In this there is nothing surprising, everything can be.
You are right, most of the time if bitcoin down alts are down too same as if bitcoin is up all alts are up too, this happens everytime and it is also depends on the activity of a particular coins why they rise.
This determine that altcoins still depends on bitcoin price which bitcoin lead them to whether the price up or down. It's still hard to understand but for deeper research you will know.