A look at the Nasdaq short interest on December 14, compared to November 30, shows that bets against discount brokers rose sharply. Short interest in E*Trade (NASDAQ: ETFC): moved up 3.9 million shares to 53.7 million, according to data from the exchange. That might have been expected, given the financial company's problems with mortgage related securities.

But, shares short in TD Ameritrade (NASDAQ: AMTD) jumped 8.2 million shares to 17.8 million, and short interest in Schwab (NASDAQ: SCHW) moved up by 6.1 million shares to 28.7 million. Both figures are a fairly large percentage increase.

The simple explanation for the rise may be that both stocks have done better than financial shares as a whole and are ready for a pull-back. Schwab's stock is up over 30% this year. Ameritrade is up just under 25%.

But there are two other possible explanations, both a bit more unsettling. One is that a bear market would likely hurt earnings at discount brokers. A recession early next year could cause individual investors to pull in their horns. The other theory is that the two firms could have balance sheet problems of their own. This is less likely, since neither company has made any disclosures to that effect.

Whatever the reason, a fairly large amount of money is being gambled that the discount brokerage stocks have peaked.

Douglas A. McIntyre is an editor at 247wallst.com.