The sell-off started with a mysterious plunge overnight that caused the exchange to halt futures
CME Group says it had to intervene with multiple 10-second pauses to prevent a steeper decline in equity futures.
Speculation for the swell in volume and plunge in futures included the news of the arrest of the CFO of the Chinese telecom company Huawei.
Traders also speculated that the selling could be attributed to a large fund or funds liquidating a position.