Moroni leaves a force in charge of the city, and then marches after the fleeing Teancum. As will be seen he will catch the Lamanite army on both the front and the rear. It would appear that the capture of the city of Mulek was complete enough that there were no escaping Lamanites who could retreat to the other Lamanite-held cities. Certainly Moroni did not want to risk the retaking of Mulek because he left it in as weakened a position as he found it. The other cities must not have know of the fall, and therefore were not likely to come to the attack on Mulek.
“Moroni Had Obtained Possession of the City Mulek”
The fortified cities were not impregnable, but the cost of life that it would take to conquer them was commensurate with the defenses left in them. The Lamanites did leave a force to protect the city, no doubt assuming that it was sufficiently large. They also undoubtedly had calculated the amount of time it would take for Moroni to move against Mulek from his previously known position, and probably assumed that they could return before Moroni could cause a serious problem.
The night march had changed that timing, however, and the Lamanites not only did not know of it, the clearly did not expect it. Their move was one that would normally have been safe, but had been made dangerous by the unusual night march. The difficulties of night marches made large scale night movements rare throughout Mesoamerica, even up to the time of the Aztec military (Ross Hassig, Aztec Warfare. University of Oklahoma Press, 1988, p. 95).