Read 1 Kings 11:27 Using Other Translations
And this was the cause that he lifted up his hand against the king: Solomon built Millo, and repaired the breaches of the city of David his father.
And this was the reason why he lifted up his hand against the king. Solomon built the Millo, and closed up the breach of the city of David his father.
This is the story behind his rebellion. Solomon was rebuilding the supporting terraces and repairing the walls of the city of his father, David.