Children often behave aggressively because they see the aggressive behavior at home, or there is something going on at home that stresses them out. Aggressive behavior usually results from fear and frustration. Children with aggressive parents or siblings will also often demonstrate aggression.
When considering behavioral psychology, children might behave aggressively as a means of escaping or avoiding a task. If the child doesn't want to complete an assignment, and he or she behaves aggressively, the child will more than likely be removed from the situation. Thus the aggressive behavior has allowed the child to avoid the assignment.
According to Glasser, humans have five basic needs: survival, love, power, freedom, and fun. If a child's needs for food, water, and shelter are met, it is almost always the case that abnormal behavior--including but certainly not limited to agression--is directly related to a deficit in one (or more) of the other four needs.
In my experience, aggression is most often related to a power or freedom need rather than fun or love (belonging).
One reason why a child might behave aggressively can be found in what is called "Social Learning Theory." According to this theory, children act in aggresive ways because they learn to do so from watching the behavior of others.
According to this theory, children see violence modeled for them by people they know and by characters in movies, TV, or even in books. Just as importantly, they see that violence seems to work for these people -- it tends to get them what they want. When children see this, they learn that aggressive actions (violence) are a good way to achieve the goals that they want to achieve. Because of this, they too act in aggressive ways.