From what I've gathered on this incident, there wasn't any wrong-doing/illegal activity that took place. Someone simply took advantage of the stop loss order function on GDAX by selling off a large number of Eth in order to drive the price down, which would then trigger the users who had set a stop loss order to sell their Eth at severely undervalued prices. Then whoever was responsible for the massive sale of Eth was apparently then ready to buy that Eth back at the discounted price to make a quick fortune.