Mormon has most certainly left out an important historical event somewhere between the story of Abinadi and this verse. When we see Noah at the beginning of the story of Abinadi, he is the powerful king and ruler of the city. He is a king who can control the labor force, embarking on large building projects. He appears, at that beginning description by Mormon, to have a fairly uniform popular support. In spite of these descriptions of a man apparently firmly in power, we have Mosiah 19:2 which appears to begin to describe a king losing his grip on his kingdom.
Somehow the uniformity that begins the story of Noah has now disintegrated into internal factionalism and a reduction of the military force. We are completely on our own to assume the causes of the internal unrest and the cause of the reduction in the military. In the case of the military, we also do not know if the reduction came because of losses in war, or due to defection to dissenting parties. We are simply presented with the stark picture of a very different Noah.
Since we have an explicit Lamanite attack in verse 6, and no indication of any previous attack, we may assume that the reduction of Noah's military was not through wartime losses, but rather defection. This reduction in the forces would therefore be directly related to the internal dissension, with those who agreed with the dissenters leaving the service of the king.
We have even less information on which to judge the internal disorder. It is tempting to relate it to the teachings of Alma who might have created an atmosphere of dissatisfaction with Noah on religious grounds. This does not appear completely likely, however, since those who agreed with Alma appear to be yet another faction, and their physical danger required them to be not only secretive, but to leave the city entirely. Thus those who remained to continue this contentiousness are probably not followers of Alma. They are also clearly no longer followers of Alma. Who were they and what caused the dissatisfaction?
It is possible that this faction (or these factions, if there were more than one) was present prior to the incident with Abinadi. Mormon may not have given us a completely clear picture of the social conditions under Noah because it suited his editorial interests to place not only Noah, but also the entire population under the light of apostasy. This gave Mormon a simpler backdrop against which to paint the story of the rejection of Abinadi and his arrest and trial before Noah and the priests.
We know that Mormon is writing the descriptions of Noah, and that he is making his own selections from history, and this allows us some leeway in reconstructing the most probable social situation. Very clearly Noah had a dominant political position, and probably statistical majority support for his religious innovations. However, it is unlikely that all of those who had followed Zeniff would have completely abandoned all of their previously held opinions and religion. Thus the presence of some in society who would have remained loyal to Zeniff's teachings is fairly assured, and this smoldering factionalism could easily be that which erupted after the death of Abinadi. It is probably that the high taxation that allowed for the building projects also created economic pressure on those who did not agree with the particular society that was being embodied in the buildings being built.
It would have been these more loyal followers of Zeniff who would have been receptive to Abinadi's preaching, and recognized him for a prophet of God. If, as Daniel Ludlow suggested, Abinadi had been one of the deposed priests of Zeniff, then those loyal to the memory and legacy of Zeniff would be particularly moved at his public and undeserved execution. While it is certainly speculative, this scenario would explain how the only events we have reported could lead to the dissolution of Noah's empire in this short of time. Perhaps this appeared so natural a conclusion to Mormon that he saw no reason to explain it. Perhaps it was simply another incident in Mormon's interest in greater spiritual stories that has left out so many things that a historian would love to know.