I think this was due to people margin-trading. Basically it means that people lend money from the exchange to buy more, in this case, ETH. But the exchange never wants to lose money, so if the price falls too low, then the exchange will start to sell the ETH to make back the money you owe.
Example:
I buy 10 ETH at 300 USD a piece. I also buy another 5 ETH using margin trading, I now have 15 ETH at 300 USD a piece, where I've borrow 5 ETH at 300 USD a piece from the exchange. So my current value is 15 x 300 = 4500 USD. But then a whale starts selling his entire position and the price plummets. In this particular scenario, if the price reaches 100 USD or below (100 * 15 = 1500 USD, which is the amount I borrow from the exchange for those extra 5 ETH) the exchange will start selling all my ETH to make back the 1500 USD I owe them. Suddenly I'm left with 0 USD and 0 ETH on my account.
What happened at GDAX seems to have been a huuuuuuge sell, I believe it filled up every buy order available, thus the price was able to be lowered incredibly fast since there was a lot on the selling side and none at the buying side. This also kicked in a ton of forced sell orders from margin-traders and simply kept going until it finally stabilised - which would be when the huge order was filled and the the buy orders had caught up again. Not really sure how the whale sold a such low prices though.
Someone correct me if I'm wrong :)