Supply and Demand at work

This large fundamental trader chose to execute this sell program via an
automated execution algorithm ("Sell Algorithm") that was programmed to feed
orders into the June 2010 E-Mini market to target an execution rate set to
9% of the trading volume calculated over the previous minute, but without
regard to price or time.

The execution of this sell program resulted in the largest net change in
daily position of any trader in the E-Mini since the beginning of the year
(from January 1, 2010 through May 6, 2010).... The Sell Algorithm chosen by
the large trader to only target trading volume, and neither price nor time,
executed the sell program extremely rapidly in just 20 minutes....

The combined selling pressure from the Sell Algorithm, HTFs [high frequency
traders] and other traders drove the price of the E-Mini down approximately
3% in just four minutes from the beginning of 2:41 p.m. through the end of
2:44 p.m....

complete report at:
http://www.sec.gov/news/studies/2010/marketevents-report.pdf