Nobody quite knows why the Nasdaq biotech index ($IBB) sank another 5.6% on Thursday. Analysts do know that the drop came the day after the index jumped 4.1%, also inexplicably, which has added to their confusion. Some have suggested pricing concerns over Gilead's ($GILD) Sovaldi may be to blame for a broader downward trend. But others say biotech stocks have simply become vastly overpriced, causing trouble in the market. Article (sub. req.)