Ethereum went down mostly because of two reasons:
1. Firstly, the introduction of a new BitMEX - ETH perpetual swap contract, which allowed traders to shorten the price of ETH with up to 100x leverage in a similar manner like the XBT (BTC) BitMEX contract.
2. The ICO sell-off factor, with many traders claiming that projects have begun to liquidate their war chests, that are often full of Ether, into fiat, leading to the current collapse of the market, not only ETH.
Ethereum keeps on getting more popular and this can be seen directly from the
eth to btc graph, which shows its progress over Bitcoin popularity.