I've heard that the cause was a string of news stories several decades ago about rare but potentially fatal allergic reactions in children that caused parents to remove children from exposure to potential allergens. You can probably already spot the irony in this because the reaction by the parents to try to protect their kids actually made them more susceptible to the problem in the first place.
It's a bit like how the news can run a story about shampoo shortages and within a week the stores will have empty shelves in the shampoo aisle. Not because there was any actual shortage, but because people all thought there was one and rushed out to stock up on shampoo, even if they didn't need any.
More generally, it's a self-fulfilling prophecy.