There is about 3 different theories towards whyh the Salem witch trials occurred.
1. The females were alone and bored at home, so they sought for attention and placed blames on innocent women.
2. The women would blame other women that they disliked in terms of revenge towards her or the family, even possibly to use as a scapegoat because they themselves were witches in real life.
3. The wheat that was commonly eaten was going through excess of watering because of the rains, causing it to mold in the inside and that was known to be a causing of hallucinations and strange body movements (which evidently caused them to looked possessed).
Number 3 is the most commonly accepted one now because even though there were witches during that era, the excess amount was bound to be caused by a group intoxication from the food.
The Salem witch trials were occurred because they saw that the bible says to burn and kill any sort of witchery, and that is was worthy of being rocked down. This was an attempt in following the biblical principles, but it went too far onto blaming innocent people brutally.
Hope that helps! :)