Battle of Thebes
In addition to this, news of Alexander had not reached the southern Greek city states for some time. He had been busy with the siege of Pelium and a rumour had reached them that he had died during the course of this siege. Demosthenes - a prominent Athenian politician - produced a man who claimed to have been present at the siege and claimed that Alexander was dead. Alexander had, indeed, been injured during this siege, so it was not a totally implausible claim to make.