In this verse, Mormon confirms that falling to the earth represents a reaction to the spiritual experience, not just the murder. Such “falling” is a culturally accepted response to the Spirit and becomes evidence of the Spirit in action. For example, contemporary Saints use other gestures to which spiritual meaning is attached. If someone folds her arms and bows her head, our cultural vocabulary assigns the meaning of prayer. Similarly, falling down is a gesture communicating that the person has been overcome by the Spirit. (See commentary accompanying Alma 29:16.)
As Mormon tells this story, he stresses his underlying message: Nephi is a true prophet, right not only about the murder but also in his other prophetic statements. It is these other statements with which Mormon is truly concerned, so he highlights their veracity at this point in his narrative.