There is a bunch of investors called “short-sellers”, who do actually the opposite. Those type of investors borrows a stock from market through a lender for higher price, then sells stock back to the market. Short sellers are betting that the stock price will drop in price. Please follow this link to get an understanding what short selling is. Some news happens that creates optimism in a particular stock. It is obvious that, shares rise in response to a good news, which is basic in economics, where the demand goes up it’s price goes up. As a result “short-sellers” suffer losses which could lead to a margin call, in which broker force investor with short positions to either deposit money or close out their positions. Then, what happen is they actually they buy the stocks they borrowed at a higher price or keep pumping money. People normally buys the stock at the current price without loosing more. For stocks with massive amounts of short-selling, creates a snowball effect, leading to tremendous volatility and huge spikes in the share price.