Here's why:

* Backward rolling motion: This refers to the movement of the rock particles in the opposite direction to the wave propagation. Imagine a wave moving to the right. The particles at the crest of the wave will be moving to the left, and vice versa.

* Side to swaying motion: This motion is caused by the wave's energy being transferred to the rock particles. The particles oscillate back and forth in a side-to-side manner.

Important Note: While this type of wave motion is common in sand, it can also occur in rocks, especially if they are relatively small and loose. The presence of a retrograde wave often indicates a high energy environment, such as a strong current or a storm surge.