Fund managers and other large investors short sell stocks (bet that stocks will decrease in price) as a hedge and/or for speculation. However, if the price of the stock goes up too fast, the short sellers get trapped and have to cover the short position by purchasing the stocks outright. This causes upward pressure on the prices of stocks.
Here is a list of stocks with the highest percentage of short positions compared to the number of shares eligible for trading. Note: The price is unpredictable and the stocks can come down just as fast as they rise.