Its the spring that actually moves and causes the bouncing, but the shocks essentially dampen that motion, both on the compression and the rebound of the spring's movement. So if the shocks are worn out or don't have enough dampening capability, then the bouncing will be more prevalent. The stiffer the spring, the more bouncing there will be, too. The stiffness of the spring should be somewhat matched to the dampening of the shock. Hope this helps.
Will