According to multiple sources including CNBC and Bloomberg the halt in Ethereum trading at Coinbases's GDAX exchange was the result of a whale selling several million dollars worth of Ethereum. This caused the market to drop significantly and resulted and several automatic stop loss trades pushing the market further down to 10 cent USD. Trading o Ehtereum was halted and resolved as of Thursday June 22, 2017. This was all confirmed by Adam White, Vice President if GDAX of Coinbase.