Pretty much all altcoins do not have a fiat pair (altcoin/USD). So what happens when the market turns red and people want to get into fiat to sit it out: they'll have to sell their alts for BTC in order to get the fiat. So when BTC/USD goes down, altcoins/BTC goes down and the dollar value of your altcoins will drop more than the dollar value of BTC.

This works in reverse too, when BTC/USD goes up steadily altcoins/BTC will go up and the dollar value of your altcoins will rise more than the dollar value of BTC.