These verses tell us a little more about Mormon’s interest in his records. Later in his editing he will not give this type of detail for a year in which there were no real actions. In this case, he is still early in working with the annalistic change in the large plates of Nephi. He gives the year, and some of the important information for the year.
In this case, nothing new happens. The information of what happens in the sixth year is the result of the wars of the fifth year. Those conflicts shook the Nephites and they realized that they must not have been sufficiently righteous. Without specifically invoking the promise of the land, that is clearly what is happening here. The wars were devastating and, therefore, the Nephites had not been sufficiently protected; they needed to repent and become more righteous. They did, and “many were baptized in the waters of Sidon and were joined to the church of God.”