I think it was a combination of his own falling out of line with his constituants - unfortunately cause I admire him so much - and the Dems nominating a familiar name and alternative.
Being from PA, I think the whole school internet thing turned people off too especially in Western PA. I just heard alot of complaints about that while visiting my grandparents hometown. Fair or not people did not like Pennsylvania giving him a "check" so his kids could go to school in Virginia regardless of whether he paid taxes or not.