Remember that for nearly a hundred years after the play was written, people believed in witchcraft enough to hang people (in England) or burn them (in most of the rest of Europe) on suspicion of it. People would have been as unnerved by the incantations of the witches, as they would be today by a scene in which a character reads out a recipe for a terrorist bomb.

In a climate of belief even more simplistic than present day America, they would have expected an ever- vigilant and vengeful God to visit nemesis on them, and it's in the nature of superstitions to count any untoward occurrance as confirmation.