Your leg moves within certain angles of displacement - back and forth - with respect to your torso (which the leg is "mounted to").
On the other hand, the angle the ball will go (the hight above the court) depends on the angle your leg assumes at the last moment of contact (the greater the angle, the higher above the ground the foot goes and the higher the ball will go).
Now, if you lean back, the same movement of your leg - wrt. your torso - will result in your foot going higher above the ground (notice that if you are just lying back on the ground and kick the ball, you will find it impossible to kick a ball in parallel to the ground unless you kick it with only the tip of your shoe - if you kick the ball with the upper part of your foot the ball will always go up).
To sum up - leaning back changes the angle of your torso with respect to the ground, which changes the hight your foot will naturally go when kicking the ball, which makes it go higher.
By the way - how high the ball goes depends also where, relative to your body, the ball is at the moment of kicking. If it is just next to your leg you are standing on, it will tend to go low above the ground, but if it is further in front of you it will rather go higher. This is so for the same reason as your initial question - angle.