Wounded unit looks for base to repair at, it picks closest one which just happens to border an enemy city.The wounded unit travels to the barracks and upon trying to enter the hex and the barracks,triggers the enemy garrisons in the adjacent enemy city.
With the enemy garrisons out,the zoc of control is active, and the wounded unit cannot enter the barracks it traveled to reach
The wounded unit is often killed right then and there,but sometimes it survives and escapes,only to retry to enter the barracks .....