It's the cliff thing. Today is a nice price to sell in preparation for the cliff. For those of us prepared to ride through the cliff, or are hoping the DC clowns can come up with a nice paraglider to take us off the cliff, this is just an opportunity to buy.
Sure looks like one of those 'sell on the news' days. best guess is people looking for an exit by the end of the year, so they don't owe more on capital gains next year. With good news like this they knew there would be buys and they could exit without losing too much. I cannot think of another reason.
The data wasn't a "home run", but on balance was positive. SGEN is building a valuable franchise against intractable disease processes. Breakthroughs tend to come incrementally. SGEN keeps demonstrating proof of concept.