Simpler than that, I think. This here modern Age of Reason holds that people always prioritize the Capital-T Truth as derived from Science and Inquiry but any behavioral psychologist or economist will tell you that we do what our friends do, we do what our idols do and that we value conformity over correctness. You stop believing in vaccines two ways: (1) you have doubts and then become exposed to an economy of doubts. That economy then becomes your worldview (2) a prominent member of your social network has doubts and you follow them in their doubt. En masse you are all vaccine skeptics. 2004: Republicans have, since capturing the Dixiecrats in 1964, prioritized effect over cause. The Party of Lincoln was more than happy to absorb the segregationists angry over LBJ's Civil Rights Act if it meant a win in '68. Very, very few of them had any problems with backing a populist with no grand policy ambitions so long as it allowed them procedural control. Republicans have watered down their beliefs for fifty years and have traded philosophy for fervency. The rise of the Tea Party demonstrated that the Republicans have no basic principles other than "be angry" which has caused them to embrace outrage regardless of the reason. You and I, my friend, have watched the two political parties devolve into "principle, regardless of the effect" and "effect, regardless of the principle" for our entire lives. I believe this is the reason Democrats are paralyzed while Liberals are outraged: the operative Democrats have seen this coming for years while the performative Democrats insist Al Franken needed to go so they could maintain the moral high ground. Robert Putnam put his finger on the problem in 2000: society was moving away from community and "community standards" are impossible without community. If your social interactions are with a cherry-picked cadre of crazies who believe Hilary Clinton drinks babies' blood, you will favor that splinter faction of crazies you've never met over your neighbor. The unofficial motto of NextDoor is "I am suspicious and disdainful of everyone around me". How does NextDoor get people to sign up? Click this button and we'll spam postcards to your neighbors. Not "hey go knock on doors you fucking hikikomori." WWG1WGA, mutherfucker. Goddamn moose lodge for the new millennium. There's no hijacking here. There's just a fundamental dissolution of community and shared identity. 
That's an excellent point, to which I will add that humans are herd/pack animals, and if the community is dissolved it will need to be replaced with something to prop up that animal need. Enter social media. Yeah, this is "old man yells at clouds" territory, but the architecture of our social media tools is such that it allows one to yell and rant in a neighbor's face without repercussion. In person, the only people who would do that would be actual sociopaths, who then self-isolate and write long manifestos on typewriters and mail them to other weirdos. Give them the megaphone of social media, and now duplication/dissemination of their message of social misanthropy enters the public square. ... which bring us back to your anti-vaxxer analogy; here's the "maverick" who appeals to your sense of injustice and intellectual confusion over why life is so hard/complex. The two loonies find each other more easily and replace that missing sense of community with their neighbors, with a performative form of 2-dimensional "community" with a group of internet randos with unclear motivations/issues/diagnoses... Social media is The Problem, due to the architecture of the thing and its design to give the brief dopamine rush in place of the real animal-need of human connection to community. There's no hijacking here. There's just a fundamental dissolution of community and shared identity...
Thank you for putting that sentiment into those words. That really resonates and helps me frame something in my head. due to the architecture of the thing and its design to give the brief dopamine rush in place of the real animal-need of human connection to community.