Thebes was a constant thorn in Philip's and Alexander's sides. When they finally revolted openly against Alexander, he marched southwards and collected number of other Greek allies (notably the Plataeans and Phocians - long-time enemies of the Thebans and victims of perceived war crimes in previous wars) and defeated the Thebans. On advice from his Greek allies (though note who he had with him... ), Alexander razed the city and sold the citizens into slavery. Would have been a pretty strong message to any potentially rebellious states, really...