Although many things in this world had changed, some events were still bound to happen. That was determined by their respective natures and Great Daos.
The conflict between Yuanshi and Tongtian had existed ever since their birth. Though Wu Chu's intervention had eased the relationship between the brothers considerably, they still bickered at most, neither willing to concede to the other—especially after they had both become saints.
Think about it: they had both become saints through Dao principles the other had comprehended. That meant both paths were viable, so why insist on deciding which was superior? There was no way to decide.
Thus, Tongtian was deeply displeased with Yuanshi's overbearing manner. Just because he was the younger brother did not mean Yuanshi could bully him however he pleased!
You walk your path, I cross my bridge. Wouldn't it be better for everyone to mind their own business?
Yet Yuanshi simply found Tongtian disagreeable, especially when he saw the disciples Tongtian had taken in. He disliked even Duobao among them.
As a result, the disciples of Chanjiao also looked down on the disciples of Jiejiao in many ways. As they spent more time together, conflicts between the disciples steadily piled up.
This particular conflict began with a discussion of the Dao among the disciples.
Strictly speaking, it had something to do with Wu Chu as well. After Duobao came to study at First Academy, he was influenced by Huanglong to some extent.
As Jiejiao's chief disciple, Duobao kept far stricter control over the disciples below him than he had in the original Primordial World mythology. He had begun to look somewhat like a proper eldest senior brother.
Precisely because of this, Duobao kept the conflict between Jiejiao and Chanjiao within a manageable range. It did not descend into the same chaos