What started the suspicion of witchcraft in Salem?

The suspicion of witchcraft began in 1692. Betty Parris and Abigail Williams, both young girls, began having fits that involved convulsions, contortions, and uncontrollable screaming. A doctor was called to consult and could find nothing physically wrong with the girls, leading to a diagnosis of bewitchment. Making the situation worse, it came out that Tituba (a slave owned by Samuel Parris) had tried her traditional remedy to cure the girls prior to the doctor's visit. This home remedy was itself deemed to be witchcraft, lending credence to the doctor's diagnosis and the girls's later accusations against Tituba. The young girls also accused a beggar-woman and an elderly, impoverished woman, both of whom were ill-thought of in the community.
While the above notes the immediate cause, there were a number of proximate causes that likely influenced the hysteria which followed these accusations. England and France were at war in North America at this time, with France allying with a number of Native American tribes. The resulting destruction of the war led refugees to flee from Maine and northern Massachusetts, leading to the increased burden of incorporating these refugees in Salem. Salem itself was also divided, with Salem town and Salem village asserting different values with regard religion and desires: Salem village relied largely on agriculture and wanted a stricter adherence to the requirements of entry into the Puritan church, while Salem town relied on the wealth connected to the port and wanted greater acceptance of the half-way covenant. There was also a significant division in power between the Putnams and the Porters, which was related to the divisions between Salem village and Salem town.
These divisions, along with the flow of refugees and the fear created by the nearby war, caused an environment that was conducive to a mass hysterical event. It was helped along by officials who showed more interest in power than in finding the truth. While the young girls who first complained of fits were the immediate cause of witchcraft charges, it was likely that accusations would have come from another individual if the girls had not started having fits.