Textual: The similarity between this account and that of Saul is that there is a falling to the earth, and all hear the voice (see Acts 9:4, 7).
The difference is that in the Book of Mormon account all have fallen to the ground, and all see the messenger (see verse 18). Since Alma the Younger and the sons of Mosiah have all heard the voice, and assuming that they are all adherents to the Mesoamerican religion, they would all have experienced the thunder and tremblings that marked this occasion as divine.
In the Old World example, the companions heard a voice, but that does not mean that they understood it at all, let alone assumed it to be divine. In keeping with the nature of the Book of Mormon event, all would have fallen to the earth as each of them would have perceived the markings of the divine appearance, and would have fallen to the earth before the majesty of the appearance.
Social: It is possible that there was a cultural expression of humility before god that involved prostration on the earth. While these men might have fallen because the earth shook, it is more likely that they fell in response to the spiritual power of the event, not the physical power of the event. Note that after King Benjamin’s powerful address, the people also prostrated themselves:
Mosiah 4:1And now, it came to pass that when king Benjamin had made an end of speaking the words which had been delivered unto him by the angel of the Lord, that he cast his eyes round about on the multitude, and behold they had fallen to the earth, for the fear of the Lord had come upon them.
The fear of the Lord caused Benjamin’s people to fall to the earth, and the astonishment of Alma and the sons of Mosiah cause them to fall to the earth. In both cases, it is probable that it was the spiritual power of the moment, and their unity of action reflected a common cultural response to that kind of divine presence.