Much of the impetus behind the witch trials grew out of the incredibly strict nature of Puritan society and the trappings of it that influenced much of the culture in Massachusetts and other areas of the newly colonized American continent. Because there were such strict laws governing everyone, anything outside the norm was seen as suspect or dangerous.
Many people began to use the excuse of a witch trial to settle old scores or to attack people that they saw as different or had some other argument with. It became a way for societies to attack things they didn't agree with or for someone to hide behind the idea of someone else being a witch in order to carry out a secret grudge or vendetta.
The witch hunts occurred in Salem because the town was naturally fearful of the unknown. Being a religious-forced town, and therefore believing in the devil attributed to the citizens' recognition of the ambiguous witches which apparently roamed their town.
Some people felt scared that they should be harmed by the witches that they believed were present, whilst others used this belief to conjure up ways to destroy their enemies and become seemingly greater people- or have more land.