S&P 500 falls 1.7% for its worst day since May, Dow sheds 600 points

U.S. stocks began the week deeply in the red as investors continued to flock to the sidelines in September amid several emerging risks for the market.

The S&P 500 fell 1.7% to 4,357.73, posting its worst daily performance since May 12. It was a broad sell-off with each of the main 11 sectors of the benchmark registering losses. The Dow Jones Industrial average lost 614.41 points, or 1.8%, to 33,970.47 for its biggest one day drop since July 19. The tech-heavy Nasdaq Composite dropped 2.2% to 14,713.90.

One optimistic sign from Monday's rout: The Dow closed well off its session low. The 30-stock average was down 971 points at it low for the day.

There were a number of reasons for the sell-off:

• Investors fear a contagion sweeping financial markets from the troubled China property market. Hong Kong equities saw a big sell-off during the Asia trading session on Monday. The benchmark Hang Seng index plunged 4% with embattled developer China Evergrande Group on the brink of default.
• The Federal Reserve begins a two-day meeting Tuesday and investors are worried the central bank will signal it's ready to start pulling away monetary stimulus amid surging inflation and improvement in the job market.
• Covid cases because of the delta variant remain at January levels as colder weather approaches in North America.
• September has the worst track record of any month, averaging a 0.4% decline, according to the Stock Trader's Almanac. History shows the selling tends to pick up in the back half of the month.
• Investors are also concerned about brinkmanship in DC as the deadline to raise the debt ceiling approaches. Congress returned to Washington from recess rushing to pass funding bills to avoid a government shutdown.

Monday's sell-off briefly pushed the S&P 500 5% below its last record on an intraday basis. It's been a long time since the market has faced a sell-off of this magnitude as investors continued to buy the dip with fiscal and monetary stimulus backstopping the markets. The index closed the session 4.1% below its record high from Sept. 2.

Stocks linked to global growth led the broad-based sell-off Monday. Ford lost more than 5%. General Motors and Boeing fell 3.8% and 1.8%, respectively. Steel producer Nucor shed 7.6%

Energy stocks tumbled as WTI crude oil fell nearly 2% on concerns about the global economy. The energy sector slid 3%, becoming the worst-performing group among the 11 S&P 500 groups. APA shed more than 6%, while Occidental Petroleum and Devon Energy both dropped over 5%.

Bond prices gained as investors sought safety. The move pushed the 10-year Treasury yield down by 6 basis points to 1.31%. (1 basis point equals 0.01%)

Big bank stocks took a hit as the falling rates may crimp profits. Bank of America and JPMorgan Chase dropped 3.4% and 3%, respectively.

"We think the mid-cycle transition will end with the rolling correction finally hitting the S&P 500," wrote Mike Wilson, Morgan Stanley's chief U.S. equity strategist. "We point to downside risk to earnings revisions, consumer confidence and PMIs."

Wilson said he believes a "destructive outcome" is looking more likely that results in a pullback of 20% or more. On Friday, University of Michigan's September consumer sentiment index came in at 71, just slightly above the August level that was the lowest in 9 years.

The Cboe Volatility index, Wall Street's fear gauge, jumped above the 28 level on Monday, the highest since May.

"We are in an information vacuum at the moment," said Jamie Cox, managing partner at Harris Financial Group. "Stalemates in Congress on the debt ceiling, worries on policy changes or mistakes in monetary policy, and a litany of proposed tax increases have dampened the mood for investors. When this occurs, corrections happen."

Stocks have struggled so far in September in line with historical trends. For the month, the Dow is off 3.9%. The S&P 500 is lower by 3.7% and the Nasdaq Composite has fallen 3.6%.

On Friday, the Dow Jones Industrial Average turned in three straight weeks of losses for the first time since September 2020. The S&P 500 saw its biggest trading volume Friday since July 19, more than doubling its 30-day average volume.

Friday coincided with the expiration of stock options, index options, stock futures and index futures — a quarterly event known as "quadruple witching." History shows volatility tends to pick up around this event.

Fed Chair Jerome Powell will hold a press conference Wednesday at the conclusion of the two-day meeting. Powell has said the so-called tapering could occur this year, but investors are waiting for more specifics, particularly after mixed economic data released since Powell's last comments.

Some investors believe this is just normal market action that can occur in September.

"The reasons for drop this morning are the same as last week: China concerns (Evergrande, regulation, COVID), Fed tapering and possible tax hikes, but nothing new occurred this weekend to justify [Monday's] declines," Tom Essaye, founder of Sevens Report, said in a note.

Other risky assets declined on Monday. Bitcoin lost as much as 10% to below $43,000.

Most commodities were in the red. Gold was among the few assets in the green, adding 0.7% to $1,764.

— With assistance from CNBC's Nate Rattner