With the reappearance of Lamanites, there is a reappearance of wickedness. Again, this is a literary device, and should not be seen as something that is inherent in all definitions of Lamanites at all times. This division occurs, and once again, the Lamanites “became exceedingly more numerous than were the people of God.” Mormon is showing a cyclical reappearance of the conditions prior to Christ’s arrival. In Jarom 1:6 we had learned that: “And they [the Lamanites] were exceedingly more numerous than were they of the Nephites”.
Most importantly, the people who are against the Nephites are sufficiently wicked that the Gadiantons are going to appear yet again. Mormon has already told us that they caused the ultimate downfall of the Nephites (Helaman 2:13–14). Therefore, they had to come back, along with all the other ills of society.