Don't know what credit default swaps are and why they matter? Neither did I. Till now. From Arnold Kling, via Reason:
A credit default swap is like insurance against default. If you want to buy a municipal bond or a corporate bond but not take default risk, you try to buy a credit default swap. You pay a fee, and in exchange for that fee the seller of the swap will make you whole if the city or corporation defaults.
The seller of swaps collects nice fees, and most of the time the borrowers don't default. But if borrowers do default, then the seller is like an insurance company in a town that was hit by a hurricane. [...]
Suppose I have sold a credit default swap on Sallie Mae. That means that if Sallie Mae defaults on its bonds, I will have to pay some of the bondholders a big chunk of money. One way I can hedge that risk is to sell short Sallie Mae securities..... However, the more short-selling takes place, the closer they get to default. It is a vicious cycle. Ordinarily, I do not believe that short-selling affects the price, but when there is massive short-selling that is driven by dynamic hedging, I can see where the short selling would drive down prices.