I offered a few possibilities that ran through my head in the post. I believe it could have been attributed to a number of things, but what really triggered it is currently unknown .. to the general public, at least.
A whale could have triggered a large sale, that led to bot traders and stop-losses executing and driving the price down more. But the amount of Ethereum that would take would be immense. I think the most plausible scenario is a software glitch.
The ethereum subreddit seems to believe it's a whale.
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When GDAX data is reliable (order books, trades) again and some time goes by to analyze it, we will find answers. If it was a whale, there will likely be some very large trades happening in a short period of time with intervals at strategic points.
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