http://en.wikipedia.org/wiki/Short_selling
Basically, the hedge funds thought the stock would go down, so they "borrowed" or "rented" the stock from someone else and immediately sold it. The idea is by the time they have to return the stock the price will have fell.
Ie, if the stock is currently worth $100, and they borrow 1 share and immediately sell it, they have $100 and owe 1 share. Later, on the date they have to return 1 share, they go and buy 1 share and give it back. If the share price then is $80 they made $20 on the transaction. If the stock went up to $120 then they lost $20.
So, they shorted the stock, because they thought it would go down, but then the amount of stock available to be bought plunged right as they had to return the stock. So, there was a huge demand for the stock right as the supply was drying up. As such, the price went very high, but they were forced to buy it anyways to due to the previous agreement (that they had to give the stock back).