The downfall of King Solomon is described in 1 Kings 11:1-13. Despite being blessed with extraordinary wisdom and having built the temple in Jerusalem, Solomon "loved many foreign women" from nations with which God had forbidden intermarriage. These wives turned his heart after other gods: "For when Solomon was old, his wives turned away his heart after other gods, and his heart was not wholly true to the Lord his God" (1 Kings 11:4). He built high places for Chemosh (the god of Moab) and Molech (the god of the Ammonites) and worshipped at them.
God was angry because Solomon had turned away from Him despite having appeared to Solomon twice and explicitly warned him against following other gods. As a consequence, God declared that He would tear the kingdom away from Solomon's son, leaving only one tribe (Judah) for the sake of David and Jerusalem.
Solomon's downfall was therefore caused by idolatry - the worship of foreign gods influenced by his foreign wives. Heavy taxation was a grievance raised under Solomon's son Rehoboam, not the cause of Solomon's personal downfall. Pride and failure to build the temple are not supported by the biblical text as reasons for his fall.
