Albert Barnes Commentary 2 Chronicles 12:2

"And it came to pass in the fifth year of king Rehoboam, that Shishak king of Egypt came up against Jerusalem, because they had trespassed against Jehovah," — 2 Chronicles 12:2 (ASV)

Shishak ... came up ... because they had transgressed - The writer speaks from a divine, not a human, point of view. Shishak’s motive in coming up was to help Jeroboam and to extend his own influence.