The events leading to Aristotles departure from the academy are a bit cloudy. Some say that after Plato died in 347 BC, Speusippus assumed the leadership of the Academy. Perhaps Aristotle quit because he disagreed with Speusippus's views, or had hoped to be named Plato's successor, himself. Another theory deals with Aristotle's old friend, Philip of Macedonia.
In 359 BC, Philip had ascended to the throne when his older brother, Perdiccas was killed fighting off an Illyrian invasion. Philip was a skillful ruler, who not only maintained peace in his own realm, but also expanded into neighboring territories. Philip captured Olynthus and annexed Chalcidice in 348 BC. Athens was concerned about the threat of an expanding Macedonia. It's possible Aristotles continuing friendship with a threatening ruler may have led to his politically charged departure from the Academy. It also appears that Xenocrates of Chalcedon left with him.