Good question:
Predestination, for Augustine, was a matter of affirming the omnipotent and omniscient qualities of God. To limit them in any way was to detract from God's glory and perfection. In other words, Man cannot alone change God?s plans for his creation.
Augustine claims that humans are not predestined to sin, and in that sense we have free will, and thus can be held morally responsible for our sins. Humans still have wills, but they are corrupted so that while there is the possibility of doing both good and evil, in actuality we will always do evil, unless one's will is healed by God's grace. It is a compatibilist notion that holds that God's complete predestination and our free wills can both be true.
In one place, for example, he rhetorically asks: ""Why, in the life of twins??in their actions, the events that befall them, their professions, arts, honors and other things pertaining to human life, as well as in their very deaths??is there often so great a difference that, as far as these things are concerned, many entire strangers are more like them than they are like each other, though separated at birth by the smallest interval of time but at conception generated by the same act and at the same moment?"" The mature Augustine had come to believe that human destiny is not shaped by lifeless stars, but by the living Lord of both stars and humanity.