According to the ancient source Diodorus, Philip was hosting a massive banquet as a farewell party before he left for Asia. Leading the procession into the theater on the second day, were thirteen statues, twelve of the Olympian gods and one of Philip. Philip wanted his march into the theater to be triumphant, and so he asked his bodyguards to stand back and out of the way to show to his people that he had nothing to fear. At that very moment, however, a man named Pausanias (not the well known geographer) rushed forward from the crowd and stuck a dagger in Philip's chest. During his escape, Pausanias tripped and fell and was killed on the spot.
Pausanias had sought revenge from Philip because apparently he, the king, and another man named Pausanias were involved in a love triangle. The first Pausanias was a handsome bodyguard of Philip's, whom Philip enjoyed very much. Soon the second Pausanias seemed to replace the first as a favorite of the king's. The first Pausanias called the second one a whore, and, with his pride wounded, the second Pausanias gave his life up for the king by taking blows meant for Philip in a battle with the Illyrians. The first Pausanias now felt slighted because he believed that Philip still preferred the dead Pausanias. To ease his pain, Attalus, a close friend of the king and leader of the upcoming invasion of Asia, got Pausanias drunk and then let several stable boys rape him. Philip liked Attalus too much to punish him, and instead gave Pausanias a promotion to a higher position as bodyguard to placate him. Doubly slighted, Pausanias plotted to kill Philip in the manner mentioned above.
Other possible scenarios include Pausanias to have been bribed to commit his crime. Olympias might have turned his mind to think of Philip as his real enemy (she could offer him sanctuary in Epiros); that the King of Persia wanted Philip destroyed (because he had sent the vanguard of an invasion force into Asia) and had sent agents to seek out an assassin; Others suggest that Alexander had asked for Pausanias to kill "bridegiver, bridegroom and bride" meaning Attalus, Philip and Cleopatra E', his father’s love-bride, and the reason for his and Olympias’ quarrel with Philip the year before.
http://www.archaeonia.com/history/hellenistic/philip.htm