The sell-off started with a mysterious plunge overnight that caused the exchange to halt futures
https://www.cnbc.com/2018/12/06/the-stock-selloff-started-with-a-mysterious-fall-in-the-futures.html
https://www.cnbc.com/2018/12/06/the-stock-selloff-started-with-a-mysterious-fall-in-the-futures.html