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.