While Cantor is an especially sleazy example, I agree with others who've noted that the problem is gerrymandering. The GOP sought to create as many "safe" districts as they could in 2010, creating hard right enclaves that they wouldn't have to defend. The problem with blood red districts is that they contain nothing but hard right voters. They've sliced out all the moderates and sent them over to form the liberal districts around the cities. So in these gerrymandered districts, the threat now comes from crazy people like David Brat. And rest assured, what people are finding from his online history is as crazy as people expected. He's like Falwell meets John Galt, only stupider.
ETA: And circling back to gerrymandering, he'll most certainly win the general election. Again, safe, red, crazy district.