It is not overall that they depend on BTC and ETH, but those coins have a part also for the growth price of the tokens. So if either of these two have increase its value, the tokens also increases.
In my opinion, most Altcoin follows the price of Bitcoin including ETH itself, because when the price of bitcoin decreases automatically Bitcoin holders will sell their Altcoin against Bitcoin, because fear of prices drops sharply, but if the price of bitcoin tends to be stable, it will increase the price of Altcoin when the stable bitcoin market will attract many Investors to trade on Altcoin.