I can imagine different people in the anti-vaccination movement having different psychological motives. For some of them, it may be just the "purity" instinct. Others may have studied the topic a lot, unfortunately from bad sources or with bad understanding. (The difference is that the latter could have reached an opposite conclusion if presented with different literature and/or peer pressure, while the former would always opt for "not doing anything against the nature".)

Then it is an empirical question of which ones are how frequent.