As according to my point of view the reason behind to follow the BTC by altcoins is that the BTC is the first cryptocurrency which comes into the existence.
And the holders have more trust on it as compared to the Altcoins and BTC is the market holder it has the power to fluctuates the market.
Another reason is that we compare bitcoin prices and altcoin prices in terms of dollars. So if bitcoin goes up, we see altcoin chart prices also rises, but in reality the satoshi value of altcoin may decline or remain the same but we see upwards charts because it is compared in terms of dollars.