First, many young horses have a hard time dealing with imbalance. By sitting on their backs, we automatically unbalance them. Then, it becomes the rider's job to rebalance them and teach them to carry the rider's weight optimally. You may want to ask someone on the ground to watch you to see if you lean to one side or the other. Bucking or kicking out is a common reaction.
Many horses also begin to run if they feel imbalance at the trot or canter.
If you find that your horse is reacting to imbalance, the best way to work with it is to go back to riding basics yourself. Work alot at the walk. Then when both of you are comfortable at the walk, move on to the trot, canter, etc. If you grip at the knee or calf, they dislike the constant pressure and will often kick or buck to relieve it. Try to develop a light, pulsing and very rythmic action in your leg.