An unusual move in US equity market futures this morning triggered a market intervention by CME Group, the world’s biggest futures exchange.

CME had to halt trading for brief periods “to enter the market preventing even harder moves”.

Traders are unsure what caused the move, which has left an extra bout of nervousness for already skittish markets.

The stock market futures were re-opening after a brutal previous session on Wall Street in which the Dow fell by 800 points, or 3%.

SYDNEY — The world’s largest futures exchange, CME Group, had to repeatedly halt trading in US stock futures for brief periods this morning because of violent price moves, in an unusual intervention that has stunned traders.

US stock futures reopened this morning in Asia after an unscheduled closure to mark the funeral of former President George H.W. Bush on Wednesday. Tuesday’s trade saw Wall Street selling off heavily, with the key indices down around 3% and the Dow down 800 points.

When futures re-opened in the Asian trading day on Thursday, stock futures plunged again, falling up to 1.9%. (They soon recovered and a short time ago were down just over 1%.)

It was during this opening in trade that CME Group intervened to manage the volatile trading activity, putting a stop to algorithms closing orders.

The price action caused several CME “Velocity Logic” events, which are triggers to halt futures trading when price movements move too far, or too fast in a given direction.

CME announced this morning that it intervened to prevent steeper falls in US equity futures.

A note from an institutional clearing desk in Sydney sent to traders and seen by Business Insider said:

CME experienced multiple Velocity Logic events from the open in Equity futures today. It caused several 10 second pauses where limit orders were accepted but market/IOC orders were rejected causing multiple algo slices to get rejected. The pauses allow participants to enter the market preventing even harder moves.

According to Bloomberg, markets are unsure what caused the move, with some speculating that it may have been sparked by an accidental “fat finger” trade.

A short time ago, Asian stock markets were trading lower across the board in midday trade, with Hong Kong’s Hang Seng Index down more than 2%.

The moves were part of a broader risk-off tone in Asia, with gold higher while the safe-haven Yen has performed strongly against all major currency pairs.