There are two main things which fed the madness depicted in Arthur Miller's play "The Crucible." First, the hatred and existing feuds allowed the villagers to fuel the madness in the town. Second, the fear of the unknown fueled the madness in Salem.
Many of the villagers in Salem disliked (even hated) other villagers. This hate was fueled by existing feuds regarding land disputes, differences of ideology (mostly religious), and personal issues. That said, some of the villagers allowed their hatred to rule the decisions they made regarding the accusation of witchcraft in the village.
Fear of the Unknown
Given that medical science was far from being established, doctors were very limited on their knowledge of how the brain works (let alone the manipulative nature of teenage girls). That said, if a doctor was unable to diagnosis a patient, the patient's "issue" was deemed supernatural (given mankind could not define it).
The compounding of the unknown (beginning with Betty's illness) and the rampant feuds and hatred (like the feud between Parris and Proctor) forced the madness of the many unsubstantiated accusations against the villagers of Salem.