"Pulling margin from long customers — The clearinghouses and broker dealers who finance margin accounts will suddenly pull all long margin availability, citing very transparent reasons for the abrupt change in lending policy. This causes a flood of margin selling, which further drives the stock price down and gets the shorts the cheap long shares that they need to cover. (Click here for more on Pulling Margin)."

http://counterfeitingstock.com/CS2.0/CS8PullingMargin.html

Sounds a lot like Thursday and Friday, no?