India, Sept. 22 -- The S&P 500 is facing fresh pressure from rising interest rates, after the Federal Reserve unexpectedly moved toward higher rates instead of cutting them. The shift has raised concerns that borrowing costs could stay high and put pressure on stocks.

September is already a weak month for the S&P 500 historically. Since 1928, the index has fallen in September 55% of the time and has lost an average of 1.1% during the month, according to Yardeni Research.

Experts disagree on why September is often weak for stocks. Some believe investors expect September to be bad, creating a self-fulfilling pattern. Others point to seasonal factors such as portfolio rebalancing and investors returning from summer vacations.

This Septemb...