"to virtually nothing" - I believe this is misreading the chart. I had to take a second look myself. The light red column at 5:10pm is trade volume (240K sold). The price stayed between $17-18. The sale of 260k at 5:50pm was priced between $10-$13. The lowest price during the 'crash' is $10.

My theory is there are miners with large amounts of BTC and someone decided to sell a lot in a relatively small market. According to the depth-of-market calculator and there is only about 2000BTC in total on the buying side right now. Its easy to swing the market.