The division of the family is no orderly, polite separation. The Lord's warning to Nephi that his brothers once again seek his life causes Nephi to gather those who are of a similar heart to his and to flee. Nephi had been in this situation before, and had never left. In this case the situation has changed, and Nephi leaves.
It is interesting that there were no angels protecting Nephi at this point. Where Laman and Lemuel had been prevented from harming Nephi in the past due to the protection of the Spirit (see 1 Ne. 17:48) the only protection afforded at this time was the warning to go. Of course the warning had the same saving effect, the outcome allowed the disintegration of Lehi's family, were before it was preserved. Presumably, the Lord knew that the time had come for the separation to occur.
Historical Analysis: Verse 6 gives us the original makeup of the "Nephites." Nephi, Sam, Jacob, Joseph, and his sisters and their families leave with Nephi along with Zoram and his family. This left Laman and Lemuel and their families as well as perhaps the families associated with Ishmael not represented by the families that went with Nephi. Sorenson suggests that there were possibly eleven adults and perhaps thirteen children in the group that went with Nephi (Sorenson, John L. "When Lehi's Party Arrived in the Land, Did they Find Others There?" In: _Nephite Culture and Society_. New Sage Books, 1997, p. 66).
This division actually suggests that perhaps the more than half of the party left with Nephi, creating an interesting demographic problem in that the later versions of the "Lamanites" have them as more numerous than the Nephites, yet they appear to have begun with a significantly smaller population. The best reading of the remarkable demographics of Book of Mormon peoples is that they intermingled with other populations that are known to have been in the area at that time period (Sorenson, "When Lehi's Party Arrived in the Land, Did they Find Others There?" In: _Nephite Culture and Society_. New Sage Books, 1997, pp. 65-104 [Note, this is a reprint from Journal of Book of Mormon Studies 1992,1:1-34]).