"Needless to say, last night's ambush was comically initiated right at the open of electronic trading, which commences in the early evening on Sunday, when the futures markets tend to be at their least liquid. There was an absolute flood of sell orders at the open but the cliff-dive chart was accompanied by a relatively small amount of total volume. This suggests that there were some motivated "sellers" trying to push the market lower and force selling by the MF Global or Ameritrade customers who would be unable to meet the new margin requirements. To be sure, there was also plenty of unloading by longs who were frightened by the volatility and wanted to protect any profits they might have."

http://jessescrossroadscafe.blogspot.com/2011/05/cme-announces-third-margin-increase-for.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+JessesCafeAmericain+%28Jesse%27s+Caf%C3%A9+Am%C3%A9ricain%29