revolt of Cyrus the Younger

At the battle of Cunaxa, two claimants to the Persian throne lined up their armies. One of them had a large force of Greek infantry, and both kings had men in their armies who went on to become famous writers. One of those aristocratic camp followers, Xenophon, tells a story which has puzzled many readers (Anabasis 1.8.19 from the Loeb). When the armies were about 600 or 800 yards apart, the Greek mercenaries ran forward:

And before an arrow reached them, the barbarians broke and fled. Thereupon the Greeks pursued with all their might, but shouted meanwhile to one another not to run at a headlong pace, but to keep their ranks in the pursuit.

It was very common in the 5th century BCE for one side to run away as the enemy approached, or after a few moments of fighting hand-to-hand. Combat is terrifying, and most soldiers of the day did not have a lot of practice working as a group. The Greeks called one such fight the tearless battle because nobody on the winning side died (Plut. Vit. Ages. 33.3, cp. Xen. Hell. 7.1.28). But it is very unusual for an army to run away before the enemy was within bowshot. What happened?