Short selling is the practice of selling shares that were borrowed from a broker and sold at a presumably higher price than the price that's expected in the near term, or at least prior to margin call. That's due to the fact that short sellers profit by buying back the same number of shares borrowed (or more) at a lower price. They then return the borrowed shares and pocket the difference between proceeds from their sales and the cost of their buys. If, however, the expectation of the lower price hasn't yet materialized, and some hot news cranks up the buying pressure, the stock will inevitably start to climb before they've re-purchased the required number of shares for re-payment. That would NOT be favorable to short sellers, since buying back the necessary number of shares required for re-payment at higher prices means they are losing money. Since the higher the pps goes the more they lose (or profits reduced), and if they fear it may climb much higher and remain above their target price range, it causes the shorters to buy frantically to minimize their losses. Add this buying by shorters on top of the already strong buying pressure and BOOM.