Shishak is the first Egyptian king to be mentioned by name in the Bible. Prior to this the Bible uses no personal names for the Pharaohs.
Because of Solomon's idolatry, God decreed through the prophet Ahijah that He was going to take ten tribes from Solomon and give them to Jeroboam, an official in Solomon's court. As a result, Solomon sought to kill Jeroboam. Jeroboam fled to Egypt where Shishak gave him refuge. By harbouring Jeroboam he instigated the division of Israel. Upon Solomon's death, Jeroboam returned and became leader of the breakaway Northern Kingdom, while Solomon's son Rehoboam ruled over the Southern kingdom of Judah.
When the split occurred, Shishak invaded the two weakened kingdoms (Judah and Israel). The underlying cause seems to have been to break Israel’s commercial monopoly in the north and to obtain much needed treasury, rather than to annex the land.