Price of altcoins are dropping because the price of bitcoin is dropping this is because the price of all coins are tied to the price of bitcoin so when bitcoin start going up in price then so will the other coins start going up in price.
I'm not entirely sure why all coins begin to grow with bitcoin, one guess that altcoins have a certain percentage of capital in bitcoin, but what I'm sure is that there are people holding pairs, for example bitcoin and Ethereum. And if one coin is bad, then there is a overflow into another coin.