There was a rumor floating around on Twitter that some traders thought Merck was interested in buying Alnylam. Thus the drop when the story broke that Merck was buying Acceleron instead.
Personally, I don't know why anyone would think Alnylam was a buyout target. JM seems to want to build the company into the next big pharma/biotech. And a lot of the current 23B market cap is tied to the value of the platform, and the expectations of future drugs from it - valuable, but speculative, not the sort of thing pharma usually goes for, not at this price range.