As has been explained, the price of altcoins is directly tied to BTC.
If you look at coinmarketcap.com, the USD prices shown are being calculated from the last ALT/BTC paring along with BTC/USD. Even if there was no trading volume on the alt, its USD price would move along with the BTC price. It takes time for investor capital to flow into the various coins after a large BTC price movement (lag).
The long-term trend seems to be that some of the capital originally earmarked for bitcoin is now making its way into the altcoins. This can be measured by subtracting the bitcoin market cap from the total market cap shown for all coins.
That number is growing and currently approaching two billion USD. At some point, the altcoin marketcap will exceed the bitcoin marketcap.