Mormon begins a new chapter with a new story. The intent of this new chapter is to introduce the political problem created when the sons of Mosiah decline political leadership in order to instead go on a missionary journey to the Lamanites. This part of the story begins here and continues through verse 10, where Mormon will interrupt the narrative in such a way that he will decide to create a new chapter to return to the essential story he begins with these verses.
The story begins with the repentance of the sons of Mosiah. That change of heart has them desiring to “preach the things which they had heard.” Specifically, they desire to preach those things to the Lamanites. Why the Lamanites? If their own nation was divided, why go to the Lamanites?
There is no answer to that question in the text. Piecing together a plausible explanation suggests that the ideas that they had previously preached, and of which they had repented, had come from the influence of the Lamanites. Thus, they may have felt compelled to preach the truth to those who had influenced their previous apostasy.