I imagine if there ever was "one religion" people gradually developed different beliefs or interpretations and decided to breakoff and start their own denomination or religion.
In other instances it may have been a falling out between religious leaders. One person wants to be the leader and can't if he/she stays in that group. This happens in the business world.
I believe God made man and man made religion!
Religion in many ways was used as a form of government to control society behavior. It established a hierarchy of power.