From the Financial Times:

Share price falls blamed on short selling return
The return of short sellers to the US market after a near-three-week ban was blamed for a sharp drop in prices on Thursday as General Motors and Morgan Stanley led stock markets sharply lower.
In another rollercoaster trading day, shares in Morgan Stanley were 16 per cent lower at $14.09 at midday in New York, while GM shares had lost 15 per cent of their value shortly before noon, falling to $5.87, their lowest level since the early 1950s.
The share price declines coincided with the lifting of a ban on traders who aim to profit from share price declines. The ban had prevented short selling on nearly 1,000 companies, including financial firms and industrial groups such as GM. The ban was lifted by the US Securities and Exchange Commission late on Wednesday night.