Usually happens both of them move together. When BTC move up most of Altcoins move up. When BTC move down most of Altcoins also moves down. But these days we saw Bitcoin moves up but Ethereum and other Altcoins didn't follow the bitcoin. There may be many ways to that happens. But I saw post say many ICOs sell their collected funds because of the dump. I believe that also affect very much to the dump of altcoins. I hope altcoin will rise again soon.