Its quite simple: People don't wanna put their money on alts. There are more sellers than buyers, so the prices is driven down. This is probably caused by the btc forks and people selling their alts to try and get "free money" when the fork occurs.
I second this. The current altcoin dump was because everyone wants to convert to BTC to get free coins from the forks. Now when BTC goes down and alts are still not rising as much as they did it's because people are exchanging to money. Things, hopefully, should go back to "normal" when the forks are done and everyone gets their free money. You should think about getting some alts while they are down.