Why are altcoins are down, it's because August 1st (Bitcoin Split). Everyone want's that free Bitcoin Cash (BCC).
If you own 1 Bitcoin, you will get 1 Bitcoin Cash. 1:1 Ratio. Similar to what happened to Ethereum and Ethereum Classic.
https://coinmarketcap.com/currencies/bitcoin-cash/
Only some exchange supports BCC:
Supported Wallets:
A good video by 
@boxmining
The split is coming - Bitcoin is very likely to split into Bitcoin and Bitcoin Cash. Find out what is happening, why it's happening and how to make sure you can claim your Bitcoin cash (and some potential risks).
Other people's thoughts
CCC from FB
However, stay safe out there!