Another likely explanation is that some whale had his bot set up with a stop-loss at $600, which would also explain why it happened simultaneously on bitfinex. Looking back at the chart minute by minute, it happened the exact moment the price hit $600.
It was not mistake, someone with inside information sold all his coins, knowing that when the real crash starts,
He did not need to have inside info, since it happened very shortly after the announcement came online. I sold manually prior to the event, and fortunately had already placed a few (small) buy orders down to $275. Thank you mr. mystery whale!