why did the witch-hunts occur?

This was a Puritan village. They were Christians who originally left England because they felt persecuted. Part of their belief system was awareness for anything "evil". They believed that witches were quite real and a gateway into the dark side, the Devil and all that. Any source of witchcraft must be destroyed, including people! So when Abigail and the other girls are caught dancing naked around the forest with animal blood they’re accused of practicing the dark arts. The girls, of course, blame other people of witchcraft to save themselves (Abigail leads the slander), the witch hunt begins!