It's probably wrong to lump these religious/political views in with the nutjobs, but like you say, it isn't grounded in reality and that's a common thread. You can't reason with them either way, because reason had no part in their views to start with.
I don't think it's unreasonable to question the sanity of anyone who sacrifices the health of themselves or their children just to 'own the libs', but I suspect a lot of it is actually just blind trust in their party who stokes their deepest fears, repeats over and over that the other party is lying/dangerous, and tells them what they want to hear. I doubt very many are knowingly taking up opposing views out of pure spite for the other team, then coming up with excuses for it, but I guess that could be what's happening subconsciously.
I think it's both and political and belief thing, and not a rational thing, as there are plenty of educated and discerning individuals who have weird beliefs that wouldn't pass any type of smell test.
As for the it being a political thing, I remember a professor who blamed natural disasters on some states legalizing gay marriage as punishment from God/nature/etc, back when that was a political talking point on TV and talk radio. I think they know, rationally, that those types of claims are not grounded in reality, but it makes them feel good to repeat them as if they were anyway. I believe that, for some people, it comes down their perceived political opponents liking vaccines, therefore they themselves will oppose vaccines to spite them, and the rationalizations work backwards from there.