John Gill 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)

And it came to pass in the fifth year of Rehoboam
In the fourth year, the apostasy of him and his people began; and, in the year following, what is next related happened, as a punishment of it:

Shishak king of Egypt came up against Jerusalem
of whom see (1 Kings 11:40) (14:25) ,

because they transgressed against the Lord
transgressed the law of the Lord by falling into idolatry and other abominable evils; the Targum is, ``against the Word of the Lord.''