Answer: The original reason for the split was the question of Muhammad's successor as a caliph
Explanation: While the Sunni Muslims had the view that successor could be anyone qualified to be a Caliph, therefore, as a religious leader who would govern the Caliphate, the Shia Muslims had a long view. The Shia Muslims had the view that the Caliph could only be someone who was a relative of Muhammad, that is, someone who was from a Muhammad's family.
In other words, the Shia wanted the principle of electing the Caliph on a sort of royal inheritance, while the Sunnis were in favor of an election appropriate for, say, the election of a president.