I guess it is done to prevent early expansion of the barbarian (or yours) factions, and i noticed it much earlier, around 260bc when i played with the Aedui. Just get a decent army (general with good morale attributes and couple of decent defensive infantry, and if possible uirodusios or something like that), stand on a bridge or a river crossing, spam them with slingers, javelins and fire arrows and they should run eventually. Just dont stand directly on the bridge, give them some space on the open ground, they will run away faster. Worked like a charm on a greek(!) general in Iuvavoeta. p.s. dont worry about the exp of their units, missiles ignore defence skill