I'm not saying that you're wrong, I just don't think there really is an "anti-incumbent" fervor out there like the media would like us to believe. I mean... find me one election cycle in which there were no incumbents in danger of losing their jobs.
Plain and simply, Specter's shiftiness finally caught up to him and Sestak handled it brilliantly. I believe that this was the last ad Sestak put out before the election. It's as near to perfect as a political ad can get, IMO. It uses footage that lets Specter hang himself.