The last time Amalickiah sent forces to war against the Nephites, they attacked from the west, and deeper north in the land than the typical invasion through the pass near Manti. That was rebuffed, and Amalickiah did not repeat that invasion. Rather, he attempted to attack again on the east, moving up the seaboard.
The eastern approach was stronger than it once had been by the defection of the Zoramites. The Lamanites had attempted that route in the invasion prior to Amalickiah’s attempt on the west, but they saw Moroni’s army and went back to the entry point near Manti. This time, the Nephite defensive armor was not a surprise, and the Lamanites had copied it. Therefore, Amalickiah attempts an invasion again.
Moroni had attempted to strengthen that region as well, but the city of Moroni was new, and perhaps not yet very large. For whatever reason, the Lamanites were able to take and occupy the city. That gave them a foothold from which they could proceed to other cities.
Because Moroni was in the center of the land fighting the kingmen, he was apparently unable to send sufficient reinforcements in time. Therefore, the Lamanites were successful.