There wasn't really a 'main' reason, there were many reasons someone could be accused on the surface level, and reasons behind the surface to accuse.

Reasons individuals might accuse another: they had a fight, they dislike them, they want something from them, a wife needs to be gotten rid of, misconceptions that led to fear, being an independent woman, being a 'hag' like woman, misfortune to an individual or community that needs to be blamed on someone. Your horse died and a crochety old woman lives next door? Must be her fault. Your uncle died of an epileptic fit and there's a confident woman next door who thinks she doesn't need a husband? Must be a witch!

When a person was tortured to confess, it was often demanded of them that they confess names of other witches. Then those people would be demanded the same. Suddenly a single 'witch' has stemmed large pools of accused persons. Children might accuse adults, or vice versa.

The church representatives pushing for witch hunts found ways to bring dissenters to trial. Even other church members who spoke against a leading witch hunt member could end up in the hot seat.

A Prince Bishop named Johann Dornheim in 1626 led a two-year massive spree of 'witch-finding' after some crops were damaged by weather. There were nearly 200 cases documented, and while some of the cases came from pure accusations, others were people who randomly named individuals under torture. The affair was cut to an end when Dornheim was set to execute a wife from a notable family and he deliberately ignored an incoming Imperial edict to free her. After he executed her anyway, the church community came down hard on him and the trials in that area were made to cease.

King James was filled with notions of witchcraft after his marriage to Anne of Denmark. When a storm delayed her ship and then their return ship, he was open to the idea witches were to blame and was eventually convinced of it. Suddenly the whole ordeal of one accused offering more accused and confessing to nonsense played out until it seemed a conspiracy of witches had set out to harm the royals.

Typically women were accused, particularly independent or old women. It was a convenient way to get rid of someone.

In 1895 Bridget Cleary would become referred to as 'the last witch burned in Ireland'. She was married but lived as a relatively independent and professional woman, a rarity for the time and area. She grew ill and her husband decided she was a fairy changeling, and with some assistance, tortured her a bit before burning her alive.

There were individuals who claimed to be authorities on witch finding such as Matthew Hopkins, who called himself 'Witchfinder General' and offered to root witches out in exchange for money. He managed to miraculously find 'witches' wherever he went and made accusations himself, and listened to those of others.

You could make up darn near any reason and some people would eat it up. Say something unfortunate happened in your life, point at a member of your community, and depending on the church in the area or who you told, they'd be pulled in to be investigated. There were occasions where people managed to argue effectively against the charges.