The Gadiantons were dressed to terrify. When the Nephites fell to the ground, the Gadiantons promptly assumed that they had succeeded in cowing the Nephites. Mormon carefully explains that the Nephites fell to the ground in supplication. Other modes of prayer among the Nephites do not require prostration, but apparently these desperate circumstances did. In many cultures, prostration indicates humility, an attitude that would certainly be appropriate in this context.