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Global stock markets tumbled Monday, with Dow futures’ decline of more than 600 points, or more than 1.6%, leading the way lower in the United States. The widespread selling was caused by a number of new investment risks.

• Hong Kong’s Hang Seng Index dropped more than 3% overnight on concern about a possible market contagion stemming from embattled Chinese property developer Evergrande Group, which is teetering on bankruptcy. European stocks also sank as Wall Street followed.
• Investors were already concerned about the seasonably rough month of September and the historical trend of the back half of the month being especially weak. The Dow Jones Industrial Average, the S&P 500 and the Nasdaq fell Friday and for all of last week.
• The Federal Reserve‘s two-day September policy meeting, which starts Tuesday, is also a wild card for markets. In the face of rising inflation and an improving economy, central bankers are weighing when they should stop buying bonds. Fed Chairman Jerome Powell has maintained that rising prices pressures will be temporary.

With Evergrande on the brink of collapse, analyst are warning the potential fallout could have far-reaching implications beyond China. Evergrande, which has grown rapidly over the years while buying up assets to support China’s economic boom, is now in debt for $300 billion. Evergrande owns more than 1300 real estate properties across China’s 280 largest cities. However, Evergrande warned that the company could face greater default risks due to its increasing debt.

U.S. Treasury Secretary Janet Yellen issued another plea to Congress to raise the federal debt ceiling. In a Wall Street Journal op-ed Sunday was a day when Yellen warned that the failure to act would cause a U.S. default, a new financial crisis and exacerbate the Covid epidemic. Yellen was the Fed’s chair before Powell. She said that last week the Treasury would use all its emergency resources to pay off debt incurred in October.

The intersection of all these worrisome factors led investors to sell stocks and risk assets and buy bonds. The 10-year Treasury yield, which moves inversely to price, dropped Monday to around 1.33%. U.S. oil prices fell 2% Monday. Bitcoin, which has seen some recent strength after a summer sell-off to below $30,000, dropped nearly 7% on Monday. In early trading, the world’s largest cryptocurrency was worth $44,275. In April, it reached an all-time high of $64,000.

Pfizer and BioNTech said Monday their Covid vaccine is safe and generates a “robust” immune response in a clinical trial of kids 5 to 11. They tested two shots that were about one-third of the dose used to treat adolescents. Pfizer will present their findings “as quickly as possible” to FDA and other U.S regulators. The New York Times Reports suggest that younger children could be vaccinated as early as October.

It is possible that the FDA will make a decision on who can get a third dose or booster shot of Pfizer’s Covid vaccine. An FDA advisory committee Friday unanimously recommended Pfizer booster shots for people 65 and older and other vulnerable Americans. At this point, the panel opposed boosters for general public. Pfizer shares and BioNTech shares fell in Monday’s premarket decline.

— Reuters contributed to this report. Follow all the market action like a pro on CNBC Pro. Get the latest on the pandemic with CNBC’s coronavirus coverage.