Why did Aristotle suddenly leave Athens in 348 or 347 BC? In his book The Lagoon: How Aristotle Invented Science Armand Marie Leroi says that there are at least two accounts which attempt to explain why. Here's an excerpt:
In the first he leaves out of pique. For twenty years he’s worked in Plato’s Academy. His colleagues call him ‘The Reader’, but he’s original too. Perhaps too original. Plato, with a hint of asperity, called him ‘The Foal’ — he meant that Aristotle kicked his teachers as a foal kicks its dam. Aelian, writing centuries later, tells a story that isn’t particularly to Aristotle’s credit and hints of a power-struggle at the Academy. One day the elderly Plato, doddery and no longer that sharp, is wandering in the Academy’s gardens when he comes across Aristotle and his gang who give him a philosophical mugging. Plato retreats indoors and Aristotle’s posse occupies the garden for months. Speusippus is useless against the usurpers, but Xenocrates, another loyalist, finally gets them to move on. Who knows if this is true; but it is certain that when Plato died the top job didn’t go to Aristotle but rather to Speusippus and that, coincidently or not, this is when Aristotle heads east.
In another version, politics rather than pique causes Aristotle to flee. Aristotle has close connections to the Macedonian court. Amyntas’ son, Philip II, is flexing his military muscles in the Greek hinterland. He’s just razed Olynthus, an ally of Athens, to the ground and sold its citizenry — along with a garrison of Athens’ soldiers — into slavery. In Athens, Demosthenes is rousing the citizenry to new heights of xenophobia; Aristotle gets out while he can.