Force equals mass times acceleration. The accelerating bullet provides a considerable amount of force on your body pushing you rearward. When you are shooting multiple shots, this rapid succession of force impulses can considerably reposition your chest cavity rearward which significantly alters your initial natural aiming area. Granted, being rocked back is more of a stance deficiency then a grip deficiency, but nonetheless, getting rocked back is not good. This video goes into how to recognize this issue and provides some tips on how to remedy this problem.