Ofcourse because altcoins price is connected to the bitcoin price simple to explain on that problem you hve, bitcoin is the king of Altcoins if bitcoin price go down also altcoin go down if the bitcoin upise altcoins is also uprise the price. In the market. Simple analysis and conclusion why altcoin affected for bitcoin price movement.
 
Lets compare to the fiat world currency the basis of every country is dollar currency if the doolar go down also the local fiat currency are also with an small exchage into the market.