An example of how wheel hop occurs in a leaf spring:
As the wheel spins, some of the energy is transferred into the leaf spring and it bends. When too much enough energy is transferred to the leaf spring, it can't absorb any more energy and it wants to "rebound" and release it's energy. So now the wheel is jerked a little bit and destabilized. The wheel hops, and then the cycle repeats. In bad cases, it actually continues to build upon itself (know as a sympathetic oscillation) and can actually break the leaf spring.
As Justin mentioned, traction bars are used to limit the "wind up" of the leaf spring.