[How the Islamic Schools Arose]

The answer—and Allah is the One Who grants success:
1- Beliefs: The multiplicity of the Islamic schools in beliefs is the result of the multiplicity of whims; so because of them the doctrinal schools multiplied, and each scholars of a school began to interpret from the Qur’an what accords with his school, and to explain it according to his whim.
2- Jurisprudential rulings: And disagreement in jurisprudential rulings is the result:
1- Either of the difference of the understandings of the scholars when examining the evidences and indications.
2- Or of their difference in the method, such as their difference in acting upon analogy (qiyās), and upon the implied meaning (mafhum) and the like of them.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3