When bitcoin rises, the altcoins keep the price and if when the bitcoin falls the altcoins fall even more. Why do altcoins usually follow bitcoin?
that's the great thing about bitcoin which can move the price up and down the price of other altcoins, but not all altcoins follow the up/down price of bitcoin, sometimes when bitcoin lowers the price there are altcoins that will increase the price, so don't be surprised for example bitcoin price movements are followed by many other altcoins .
There is a concept called correlation that let us see more clearly if two assets move on average in the same way even if there are moments in which they move differently, some time ago I took the time to do this experiment and with a correlation of 1 meaning that the assets are moving exactly the same way, -1 which tell us they move in the opposite way and zero which means they move independently from each other, altcoins have a correlation of 0.9, which tell us their movements are heavy correlated to the way bitcoin moves.