In the last two verses, Mormon finished the story of the Gadiantons. The flight into the north certainly took place after the thirty-first year, but in verse 14, we return again to that earlier year. Mormon is telling multiple stories, and he does so by finishing a story, and then backing up to the common point of departure. We see this clearly here, but he also used that technique to tell the stories of Zeniff’s people and the missionary efforts of the sons of Mosiah.
The story for these tribes is that they may not have been Gadiantons, but they were still not correctly following the Nephite religion. “Their hearts were turned from the Lord their God, and they did stone the prophets and did cast them out from among them.” That information sets up the next story in the following verse.