Reason: House Majority Leader Eric Cantor supports amnesty for illegal aliens.
What usually happens when a prominent Republican loses his primary: he then re-enters the race as "Independent", thus splitting the vote and ensuring the Democrat wins the general election. I don't expect Cantor to do any different, because the GOP leadership has demonstrated that it doesn't care what the rank-and-file of the party want. Considering his politics--and considering how things have been in the past decade--I'd be surprised if Cantor accepted the results of the primary and quietly served out the rest of his term.
It sure would be nice if he did, though.