Not all cults are just "started", they evolve from other religions. People just slowly branch off and decide that they have different views than those of their current preacher or religious group. They aren't started to go against or disagree with other religious groups, they are just formed so people can worship in different ways and worship who and what they want to.

In a sense all religious organizations could be considered cults by other organizations simply because they believe and worship differently than another given religion.