#1. Ethereum just experienced one of the largest runs of any coin from $12 to $400 in just a couple months!! So it is natural to see around a 50% retracement, this is based on human nature and golden ratio etc.
#2. EOS ICO is happening and many are freeing up capital to get in on that which is highly pumped and going to attract a lot of short term investment money.
#3. There are multiple weaknesses in Ethereum which other coins have stepped up to make improvements and compete. Long term it will still rise from here, but it's far from cornering the market.