The reasons for today's decline in the U.S. stock market may be related to the following factors:

1. Uncertainty in economic data: Concerns remain about whether the U.S. economy is slipping into a recession. Although the Federal Reserve indicated in recent meetings that it would focus on the labor market and consider interest rate cuts, uncertainty about future economic prospects may have dampened investor confidence, thereby affecting stock market performance.

2. The Federal Reserve's monetary policy: The Fed announced a 50-basis-point rate cut at its September meeting, the first cut in four years. While rate cuts typically boost the market, Fed Chair Jerome Powell stated during the press conference that the Fed is in no hurry to further ease policy and does not believe the era of cheap money will return, which may have disappointed the market.

3. Impact of the global economic environment: Uncertainties in the global economic environment, including trade tensions and geopolitical risks, may affect the U.S. stock market. For example, tensions in the Middle East could impact energy markets, which in turn could influence U.S. stocks.

4. Market volatility: After a period of gains, the U.S. stock market may experience a normal pullback. Additionally, options expiration days could increase market volatility, as the expiration of a large number of options contracts may trigger significant buying and selling by market participants.

5. Corporate earnings expectations: If market expectations for future corporate earnings are poor, or if actual earnings reports fall short of expectations, this could also put pressure on the stock market.

6. Technical analysis and market sentiment: Certain signals in technical analysis, such as stock prices falling below key support levels, may trigger panic selling among investors. Meanwhile, shifts in market sentiment, such as concerns about a recession, can also influence investor decisions.

It's important to note that stock market fluctuations are influenced by multiple factors, and the reasons mentioned above are only potential contributing factors, not an exhaustive list.