There is one part of this verse that is very clear. The presence of Morianton in the northern parts of the land would be strategically dangerous for the Nephites. What is less clear is how this was dangerous.
The first reason for the danger has to lie with the defection of the people of Morianton. With their demonstrated willingness to raise arms against members of the Zarahemnal hegemony, they indicated that their allegiance was very tenuous at best. Thus this is a potential dissident people. The location to which they were heading was apparently seen as an important area for the conceptual defense of the Nephite territory, and having that location held by dissident who had proved the propensity to hostility apparently created a scenario Moroni found intolerable.
What is probable the second reason, and the least clear, is the phrase that tells us that: “they would hearken to the words of Morianton and unite with his people.” Mormon does not favor us with the definition of who “they” were. From reading further in the text, it appears that Morianton was heading to the area of the land of Desolation, the ancient territory of the Jaredites. While we tend to assume that the land was actually empty because of the destruction of the Jaredites, it is quite unlikely that the land was devoid of peoples, and indeed archaeological evidence tells us that it was not.
The presence of people already in this area suggests two things. First, they were not hostile to the Nephites, as we do not see any wars originating from the north. Second, they were probably the people with whom Moroni feared that the people of Morianton would unit, and “hearken to the words of Morianton.” If Morianton should somehow stir up these people who had previously had no reason to come against the Nephites, then the danger to the northern cites was grave indeed. Certainly Moroni has reason to fear a dissident among potential allies, since his most recent opponent, Amalickiah, was precisely such a dissident who had gained control of the Lamanite lands and armies.
Textual: This verse gives us an example of a correction inserted in the text. The sentence begins by having the people in Bountiful afraid of the actions of Morianton, and the inserted correction changes the fear from the people to Moroni. Of course we have the same problem with this corrective insertion that we have had with the other examples of the same thing. It is difficult to tell the source of the insertion. It could have been Mormon, who began writing, and then had to change rather than erase because of the permanence of writing on metals. It could also be Joseph Smith who was dictating the text, and therefore was also in a situation where words might be expressed and be difficult to erase.