Also keep in mind that all of the alt coins (save Litecoin) are pretty much pegged to the value of bitcoin. So if bitcoin goes down by 15% all other coins will go down 15% as well. In fact, alt-coins are doing worse because cashing out is a two step process. For someone looking to sell they first sell alt coins to bitcoin (driving down the btc/alt price) then sell btc for fiat.

They are correlated assets.