With a volume of only 2 million shares traded compared with a normal 1.5 million, that would account for a very small proportion of the 11.5% of the float that's shorted. Also, keep in mind that not all shorts are betting against the stock. In a volatile or risky stock, it's not uncommon for longs to short their own stock on the peaks, cover when the dips reverse, then re-short when it hits another peak. Given that biotech/pharma stocks are subject to catastrophic falls (see the October chart of Ariad Pharm (ARIA) for an example), the short serves as an insurance policy that can also generate some short-term profits. Of course, if something really great happens (like the buyout of Onyx (ONXX)) your short cancels out the windfall profits you would otherwise realize. Sometimes the
strategy works, sometimes it backfires.