Erie County is Sestak country. Probably just didn't want to vote for an environmental consultant (married to another consultant, no less) from a Main Line suburb. What is the case for downballot lag helping her here?
You would think the same thing about somewhere like Cambria County, home to the postindustrial steel town of Johnstown. Trump won by 37 points, Toomey by 24 points. In neighboring Blair County, home to the postindustrial railroad town of Altoona, Trump won by 45 points while Toomey won by 39 points. The difference between the two, why Toomey lagged behind more in Cambria County, is that Altoona was a historically Republican city while Johnstown was a historically Democratic city.
As for Erie, the difference is very likely due to Toomey investing in the Erie media market, which encompasses Erie, Crawford, and Warren Counties in Pennsylvania. Both of those counties also swung heavily to Trump but Toomey ran only a point or two behind.