Traders Search for Clues Behind Biggest S&P Reversal Since April

The most dramatic intraday reversal in the US stock market since April dragged benchmark indexes to their lowest levels in more than two months — all while leaving bewildered Wall Street traders scratching their heads about what exactly caused it all.

While there was no obvious catalyst to blame for a plunge of almost 5% in the Nasdaq-100 Index from its high of the day, theories for the selloff were piling up. The losses looked set to extend Friday, with Nasdaq 100 futures sliding 0.8% by 5:05 a.m. in New York.