I'd only add that some portion of the voters are acting on an emotion, an emotion that isn't necessarily tied to a single specific (like immigration). In David Brat's win, I'm guessing that the Richmond district, who are a more savy than average set of voters due to being so close to D.C., were more upset over what they perceived as Cantor's dishonesty and a perception that he was far more focused on his ambition to become speaker then on the principles they cared about. I suspect that Boehner gets a bit more of a pass from his district because he is percieved as more honest and trying to make things work (whether there is any truth to that at all or not).
This is something that all of the establishment GOP politicians are faced with - the implicit question that if they aren't fighting for a coherent set of principles, then why are they trying to stay in office - what is it that motivates them? The Liberatarian wing of the conservatives should be asking variants of that question out loud. "If you guys take the constitution seriously, what are you doing about it?" "If you guys really want to balance the budget, what are you doing about it?" "Beyond getting Republicans elected, what IS your goal?"
I'd love to think that the Tea Party rose up and voted for libertarian principles when they gave the boot to Cantor - and in doing so signaled a nationwide resurrgence of votes for liberty, but I suspect that it was more about the dedicated few (some whose primary drive is religious, some who are obsessed with Amnesty, and a few others who had grown disgusted with Cantor's dishonesty) were motivated to go out in the rain, while the rest of the district either didn't care, or thought that Cantor had it in the bag, so why get wet. It was a big win percentage wise, but the actual numbers voting was very low. However... I'm happy to have the new blood and happy to have the establishment GOP scared that each and every one of them could be next.