S&P 500 Peaks
Another reason may have been accelerated business spending in preparation for the Y2K switchover.
Once New Year had passed without incident, businesses found themselves with all the equipment they needed for some time, and business spending quickly declined. This correlates quite closely to the peak of U.S. stock markets. The Dow Jones peaked on January 14, 2000 (closed at 11,722.98, with an intra-day peak of 11,750.28 and theoretical peak of 11,908.50) and the broader S&P 500 on March 24, 2000 (closed at 1,527.46, with an intra-day peak of 1,553.11); while, even more dramatically the UK's FTSE 100 Index peaked at 6,950.60 on the last day of trading in 1999 (December 30). Hiring freezes, layoffs, and consolidations followed in several industries, especially in the dot-com sector.
The S&P 500 is a market free float-weighted index published since 1957 of the prices of 500 large-cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock market companies; the NYSE Euronext and the NASDAQ OMX.
Wikipedia: S&P 500
Wikipedia: Dot-Com Bubble