The defiance thing itself is induced by being a half-sentient talking ape that doesnt like being told things like "No!" "Thats bad, dont do that!" In terms of their actions. The antivaxers also like to layer in government mistrust and religious faith levels of i-must-be-right but the core of defiance is just that people dont like feeling shitty and have poor instinctual coping methods for it.

Its a fairly universal copout for humans to avoid feelings of shame and embarrassment or punishment. Nobody on the planet likes feeling stupid or in the wrong. So when confronted with these feelings some common stategies are out right lying to others or ourselves, or constructing a delusional warped narrative subjective reality in which they're actually somehow in the right through mental gymnastics, or by rejecting the claim of wrongness through formal argument, or using social dominance plays to forcefully dismiss claims of wrongdoing.

Usually people find a strange mixture of multiple of these strategies in emotional combative arguments and coping with their aftermath.

In an ideal world people would be able to instantly accept blame and fault for wrongdoing, eat he feelings of shame and self improve without trying to weasle their way out of punishment if they think they can get away with it. A properly adjusted adult who has eaten their fair share of humble pie over heir life encroaches on that kind of humility and willingness to update their assumptions given evidence, is able to say "oh shit I was wrong about that, huh? My bad." Or "the science nerds who study these things probably have more authority on the matter than I do, I'll default to their ideas." This however, is not an ideal world.

Children have zero impulse control and are barely sentient enough to construct coherent sentences or internal monologues. Their tiny brains are still developing and so they get a free pass more or less when it comes to being defiant little shits defaulting to instinctual ape copout strategies. all kids have their bad moments and are still learning. The problem is when they never stop defaulting to these things in adulthood.

I don't see anyone dealing with it.