Because people sell watching other guys are selling(the market in general or a stock specific). The price movement of a given stock in the market is highly dependent on past price movements of the stock and other stocks. In a mathematical terms the movement obeys non-linear dynamics with great deal of positive feedback components. One result of such dynamics is that it exhibits hysteresis particularly for biotech stocks which have no earnings; the price does not spring back right away as the rest of the market recovers.
--Despite the touting of an obvious ISIS shareholder (see above)---one reason for ALNY's stock price drop--might be its poor E.R., released this week. BUT--generally--ER's in biotechs have NO meaning--since there, by definition, are NO earnings. Ergo--if you think ALNY's research is promising (see their wew site)---then a BUYING OPPORTUNITY has arisen.
My earlier post about the ISIS/BMY deal, and its implications for ALNY (e.g. ALNY is "Not Ready for Prime Time") does have some VALIDATION! - - from the ISIS Board posted 5-11-07:
"i spoke with a broker from morgan stanley, i respect his opinion, i asked him about the issue you talked about in your post (ISIS/BMY Deal), he said your correct. that is the talk around the office. only time will tell, however, it sounds positive (for ISIS).
Judge for yourself - - but EVERYTHING I have posted about ALNY's slide is supported by FACTS. ALNY has REAL competition from ISIS, and ISIS is YEARS ahead with its product pipeline (17 drugs). But here's the real Kicker - - the ISIS drug partnered with BMY (PCSK9) for a potential $200 million deal was in PRE-PRE CLINICAL - - so BMY's decision was based on the ISIS Antisense Science, and the ABILITY of ISIS to bring this drug successfully to the market. It was a BIG VOTE for Antisense OVER RNAi - - IMO! AND the market agrees!