Tanger Factory Outlet Centers (NYSE: SKT) has performed quite poorly lately. After a 7% drop following its first-quarter earnings release, the stock has now underperformed the S&P 500 by a staggering 69 percentage points over the past two years.Apparently, investors don't think the worst is over just yet, as more than 22.5 million shares of the retail real estate investment trust are currently sold short, representing 24.4% of the company's outstanding shares.