This is not a new development, but a continuation of a longstanding dispute between Nephi and Laman and Lemuel. Even while Lehi lived, the brothers had disputed with Nephi and even sought his life, but upon the death of Lehi, the last vestige of familial loyalty was disappearing, and the undercurrent of contention rose quickly to the surface.
It is most important that Nephi recognizes that the problem is not simply a conflict over a stolen primogeniture, but the consequence of lives made uncomfortable by the constant reminder of the Lord and a Spirit that they had actively denied. Once a person has turned against the Spirit, its presence in another person is uncomfortable, and one of the common reactions is anger with the person that causes such feelings in the heart of the one who has rejected that Spirit.
“Because of the Admonitions of the Lord”
Nephi explains that the source of the contention is not anything new, as some of the things that bother Laman and Lemuel were words of Lehi - obviously spoken when he was alive and therefore not any new development.
Textual Analysis: Nephi also provides some more information about the construction of the texts that comprise his record. We know that the large plates were the beginning of his record, as Nephi tells us that the small plates are begun after the large plates (1 Nephi 9:4-6). As it is unlikely that an historical account (as Nephi reminds us is the purpose of the large plates) would be considered "finished" it is most likely that Nephi writes in both sets of plates basically concurrently. Neither is considered "finished" so that the writing in the other might begin. Nephi is keeping a parallel record.