“What causes church splits?”
Let’s start with this proposition: God is gracious, and may continue to bless the efforts of his people even when they make mistakes, often in spite of them. But I think we can safely say the Lord is never behind factionalism. Even Martin Luther worked to reform Roman Catholicism from within for fifteen years before settling for the alternative.
In short, there is no such thing as a good church split. Some other outcome is always preferable, and something irreplaceable is lost in every fracture of a local testimony.