Altcoin prices mostly depend on bitcoin. Even if the trading pairs of the coin is in ethereum, the price of ethereum depends on the price of bitcoin which means that the prices of coins depends on the price of bitcoin whether directly or indirectly due to trading pairs. That is why you see that when the price of bitcoin falls, then the prices of all the other altcoins in the market start falling too.
Can't agree more! Obviously Bitcoin is the flagship cryptocurrency and even Ether derives its price from it, so even if the price of an altcoin is quoted in Ether, it indirectly goes to depend on the price of Bitcoin. An exception though can be found with Ripple and I suppose that can be explained by the fact that Ripple is not your regular cryptocurrency and for purpose like above, it was deliberately made to be centralized against the decentralized nature of crypto currencies.