Apparently, there is also a strong connection to belief in bodily purity--people who preach about toxins are more likely to fear vaccines, because you're putting all that nasty stuff in your body.
Wakefield, of course, was motivated by profit. Jenny McCarthy, I believe, is motivated by three factors--for one, a genuine need to understand why she has an autistic child. (Who she now believes is cured, because yeah. That poor kid.) For another, a need to feel smart, especially given how frequently people have called her stupid over the years. And for a third, a need for attention. She'd faded from public view until she hopped on the anti-vaccine bandwagon.