Mormon’s description of the passage of the years has been leading up to the eightieth year. In that year there is a significant change. Dissenting Nephites have gone to the Lamanites, and stirred up trouble. This is not a new thing. It has happened multiple times, and Mormon has made it clear that the most devastating wars with the Lamanites came at the hands of dissenting Nephites.
What is different here is that Mormon attributes this new threat to “robbers of Gadianton.” In Helaman 11:10, the secret plans of the previous Gadiantons had been buried. These new Gadiantons seek them out. The connection to other groups named Gadianton is through the ideas and methods they employ, not any direct connection among the groups.
In this case, the new apostate-driven Lamanite incursion begins as more of a guerilla-style warfare, or perhaps simply raids to acquire goods. As time passes, however, they become “an exceedingly great band of robbers,” and then they become dangerous Gadiantons themselves.