Altcoin investors are pulling out their coins and reinvesting it on bitcoin for the purpose of obtaining bitcoin gold. That diminishes most of altcoin's market cap making their price low. Don't worry though, after the fork prices would pump again like crazy.
You nailed it 
Not only investor are pulling out their money but also the traders. They invest or trade their money for bitcoin thus altcoins price go down.The Dash price rose 16% today!