Mormon may have recorded the battle out of sequence, since he reports the result of Lehi’s attack in the rear before he tells us what happened with Moroni’s detachment on the other side of the river. In any case, the Lamanite army, though under attack, crosses the Sidon toward Manti. Here, however, they meet Moroni and his detachment.
The front ranks of the Lamanites will be slowed as they emerge from the water and begin to climb the farther bank, even if the slope is gentle. They will also be hampered in forming a battle line. The unit hiding on the west side of the river has a tactical advantage in holding higher ground (the river banks must be lower than the surrounding lands) and in its ability to create a wider front.
The Lamanite army is now actually fighting on three fronts: against Moroni’s unit on the west, against Lehi’s unit on the east where they must turn about to defend themselves, and in the center, against the river itself.
The result of the dual pressure on the west and east banks was that the Lamanite army attempts to break from the fighting and move toward Manti. Presumably, any escape south and out of the land of Zarahemla was effectively blocked. Their only choice was to press deeper into Nephite territory and hope to gain a foothold in a defensible location.