There were an unusally high number of short sellers.
This story is from Dave Carpenter, AP Business Writer:
The Chicago exchange trades options on the stocks of about 1,400 companies along with 38 stock-based indexes, including the Dow Jones Industrial Average, the S&P 500 and the Nasdaq 100.
All nose-dived in the aftermath of the attacks, which would have meant substantial profits for anyone who had bet on their decline by buying put options or through short-selling. Short-sellers borrow stock and sell it in anticipation of buying it back later at a lower price.
In the days before the attacks, unusually high numbers of put options were purchased for the stocks of AMR Corp. and UAL Corp., the parent companies of American Airlines and United Airlines, which each had two planes hijacked. There was no such trend involving other carriers.
On Sept. 6-7, when there was no significant news or stock price movement involving United, the Chicago exchange handled 4,744 put options for UAL stock compared with just 396 call options - essentially bets that the price will rise. On Sept. 10, an uneventful day for American, the volume was 748 calls and 4,516 puts, based on a check of option trading records.
On Monday, the first day of trading following the attacks, shares of AMR fell 39 percent and UAL stock plunged 42 percent. Other airline shares also were sharply lower and most rebounded modestly Tuesday.
According to a report in The Wall Street Journal, the SEC said it had received information from various U.S. agencies Friday about possible trading by terrorists in industries affected by the bombing, including insurance and the airlines, and also about possible put-option or futures-index trading.