As we have seen, there are two parts to any of the named cities in the Book of Mormon, echoing the similar pattern throughout Mesoamerica. There is the city proper, and there is the land that is attached to that city. In this case, we have a new city, Lehi, that has been built within the last four years (Alma 50:15). Morianton was probably already in existence at the time of the construction of the city of Lehi.
When the city of Lehi was created, it would require an attached “land” so that there would be fields available for the growing of the crops that would support that city. As an adjacent land, at least some of the land attached to the new city of Lehi appears to have been taken from lands that the city of Morianton had claimed as their own territory. While they may have been willing to give up these lands as part of the need to build defenses, something that was certainly on their minds during the Lamanite wars, the years of peace appear to have allowed them to change their minds.
Note that this verse very specifically notes that the claim was on part of the land of Lehi, not the city itself. This is a border contention. The next thing that we should learn from this incident is a reinforcement of the concept of the loose hegemony of Zarahemla over its beholding cities. These city-states are able to make decisions on their own, and not consult Zarahemla. This pattern is precisely the one seen in the city-states in Mesoamerica where the loose coalition could be threatened by internal squabbling.