From the Wall Street Journal's MarketBeat blog:
...but there may be another powerful force at work keeping the markets down. Ashraf Laidi, chief foreign exchange strategist at CMC Markets US in New York, wrote in a note this morning that today “marks the last day of the 45-day period by which clients of some hedge funds are required to give notice to withdraw their money.”
... “The S&P 500 and Dow may shed 1.7%-2.0%, or up to 25 and 180 points respectively today,” Mr. Laidi wrote.