The S&P 500 ends its longest calm period in over six years, falling more than 1% for the first time in 53 trading days as AI trading loses momentum

The longest calm period for US stocks has ended, with tech giants leading the decline. The market is simultaneously pressured by inflation and credit risks.

The S&P 500 Index fell 1.1% on Wednesday, ending the longest streak without a single-day drop over 1% since January 2020, lasting 53 trading days. This downturn was driven by two major factors: September US CPI came in higher than expected, and growing concerns over the sustainability of AI-driven trading.

The technology sector was hit the hardest. The Nasdaq 100 Index slumped 2% in a single day, marking the largest daily loss since July; the Magnificent Seven Index, tracking the "Seven Tech Giants," posted a similar drop. Semiconductor stocks fell for the sixth straight session, with cumulative losses widening to nearly 12%.

At the same time, AI cloud computing service provider CoreWeave's corporate bonds were sold off, with the price of its 2032 unsecured bonds falling below 80 cents on the dollar—a threshold typically seen as a warning sign of financial distress. The company’s stock price consequently plunged 15%.

This downturn left quant model-driven systematic funds particularly passive. Over the past few weeks, these funds had aggressively bought stocks amid persistently low volatility, and the sudden end of the calm period directly disrupted their positioning logic.

Official data show that the Consumer Price Index rose 0.6% month-on-month in September, the fastest pace since April, making the market more cautious about the Fed’s policy path.

Chris Zaccarelli, Chief Investment Officer at Northlight Asset Management, said the inflation data reignited fears of further Fed rate hikes. "The market originally expected the September rate cut to be a one-off, but this morning's data has greatly reduced the likelihood of that scenario."

The Russell 2000 Small Cap Index, which is more sensitive to interest rates, fell 1.9%, retreating 11% from its August peak. It is noteworthy that even though treasury yields spiked and then retreated after the CPI data release, the stock market continued to fall, indicating that this selling pressure was not driven solely by rising interest rates.

Market fragility was on full display Wednesday. The CoreWeave bond crisis synchronized with the tech stock drop, leaving investors with nowhere to hide.

Matt Maley, Chief Market Strategist at Miller Tabak + Co., noted that previously, the market had seen some rotation buffer between credit risk and tech stocks: "On days when credit risk was the worry, tech stocks held firm; on days of tech sector fears, credit provided some relief. Today, both struck together."

CoreWeave's corporate bonds, according to Trace pricing data, have dropped below 80 cents. This level is typically seen as a dividing line for financial distress in the credit market, adding extra pressure to overall risk sentiment.

The concentrated sell-off in tech giants is the core feature of this correction. Among the "Magnificent Seven," only Apple closed up 0.5%, while the rest all declined.

This drop is especially unfavorable for systematic quant funds. These funds, driven by rule-based models, kept increasing positions in a period of ultra-low volatility, and the sudden break in the calm directly triggered position adjustment pressure.

Deutsche Bank data show that the positioning gap between discretionary and systematic funds has widened to the second-biggest level since 2020. The former, due to concerns over high treasury yields and geopolitical risks, have reduced stock holdings to the bottom third of the historical range.

Jonathan Krinsky, Chief Market Technician at BTIG, warned: "We have already seen semiconductor stocks falling as yields rise. Once the Magnificent Seven join in, it will further confirm our view of a 'Red October.'"

Nevertheless, the intensity of this downturn remains moderate so far. The S&P 500 Index has only pulled back 1.3% from last week’s all-time high, and the $13 trillion in market cap added since the end of March remains largely intact.

The Chicago Board Options Exchange Volatility Index (VIX), which measures market anxiety, hovered near 17, below the 20 level that typically signals market stress.

For bulls, a continued decline may attract sidelined discretionary funds. These funds focus on economic growth prospects and corporate earnings quality. During the recent rally to record highs, they largely stayed out, but if the correction continues, their willingness to buy dips may increase.