Moroni didn’t need a long memory to understand what a significant defection to the Lamanites might mean. He had just defeated a Lamanite army fortified by renegade Zoramites. He could easily see the same situation recurring, so he decided to block the Amalickiahites’ departure.
Geography: While Mormon does not give geographical specifics, Moroni’s assumption that he might “cut off” the Amalickiahites suggests that they were north (or downriver) of Zarahemla. Thus, Moroni and Zarahemla were farther south, blocking the route by which the Amalickiahites would reach the land of Nephi, which was yet farther south. The overall picture we have already seen that the downriver locations tended toward dissent seems confirmed here.