After Rehoboam's position as king was established and he had become strong, he and all Israel with him abandoned the law of the LORD. It seems to be that way with people. After God's given them something they seem to temporarily forget about him until the next time they need help, or in this case, forever.

Because they had been unfaithful to the LORD, Shishak king of Egypt attacked Jerusalem in the fifth year of King Rehoboam. With twelve hundred chariots and sixty thousand horsemen and the innumerable troops of Libyans, Sukkites and Cushites that came with him from Egypt, he captured the fortified cities of Judah and came as far as Jerusalem.
Then the prophet Shemaiah came to Rehoboam and to the leaders of Judah ,and he said to them, "This is what the LORD says, 'You have abandoned me; therefore, I now abandon you to Shishak.' "
The leaders of Israel and the king humbled themselves and said, "The LORD is just."
I think it was great of them to do, but it stinks that they didn't do it earlier.
When the LORD saw that they humbled themselves, this word of the LORD came to Shemaiah: "Since they have humbled themselves, I will not destroy them but will soon give them deliverance. My wrath will not be poured out on Jerusalem through Shishak. They will, however, become subject to him, so that they may learn the difference between serving me and serving the kings of other lands." All God's punishments are always fair. After they repented he said 'OK , I'm gonna show you what it's like to have other king rule over you so you can see what a great God I am.