I think it's only because hedge funds see biotech stocks, in particular, in this environment, in the light of
If there's not a compelling reason to buy (i.e., more news) then that is a compelling reason to sell.
I think that's why we've seen several (many?, most?) biotech stocks sell off after major (for them) positive news. Long term, this arena should make for good buying opportunities. If the economy crashes, all bets are off.