If you load the rack improperly, the load can shift and become unbalanced and cause the bike to want to rock back and forth, just as, if you load clothes in a washer all on one side, the basket rocks back and forth rapidly as the clothes circle from one side to the other. Not necessarily a knock on the rack, just load the bike so that the weight is low and the load is secured tightly.
Edit: And balance the load as much as possible left to right, and with more load on the back than on the front.
Last edited by ploeg; 12-07-09 at 08:59 PM.