According the Brant Gardner, the increasing pressure from the Lamanites suggests two plausible scenarios that would ultimately lead to the Lord's command for Mosiah1 to flee the land of Nephi:
(1) The city of Nephi would come under threat of a Lamanite attack that the Lord knew would be more effective than previous attacks either because of decreased ability of Nephite resistance. Either the Lamanites got stronger or the Nephites got weaker. Thus Mosiah1 and the righteous would be asked to flee the destruction.
(2) The need for the Lamanites to directly attack the city of Nephi might have slowly disappeared as the Nephites accomodated to the Lamanite culture. This would have been an official capitulation by the rulers (not the prophets), and although the city would have continued, the environment would have been untenable for the righteous.
[Brant Gardner, "Book of Mormon Commentary," [http://www.highfiber.com/~nahualli/LDStopics/Omni/] Omni1.htm, p. 15]