Three reasons.
1. People were flipping out of Bitcoin into Eth as Bitcoin scaling dispute just drew on and on - now Segwit 2x will activate, scaling is solved, people are flipping back. This is the most important reason.
2. Ether developers often stuck their money loving fingers into numerous what I consider to be semi-fraudulent ICO offerings (I'm not saying fraudulent as I don't want to be sued for defamation). All that Eth raised by the clever and quick in these ICOs does not want to stay in Eth, it wants to go to USD, so there's constant sell pressure to convert Eth raised in ICOs into USD.
3. There was an altcoin bubble and it's over. The whole market save Bitcoin is significantly correcting down after a huge run up.
Ethereum dump almost 20% What is the main reason why ether is dumping now please any one here explain Why ?