We have the battle perhaps out of sequence. We get the result of the rear attach by the army led by Lehi before we get the consequence on the other side of the river. In any case, the result of the battle is that the Lamanite army moves across the Sidon. This not only moves them away from the threat to their rear, but moves them closer to their goal in Manti. On the other side of the river, however, they now meet Moroni and his armies.
It would appear that Moroni placed his armies in two locations. One was an ambush side on the west of the river and probably somewhat south of the ford location. In this way, when the fighting continued after the fording the logical direction of flight would be north towards Manti, where apparently there was a large force awaiting them. Lehi holds his army from crossing the river. This move protects the Lamanite retreat zone, and does not place his army at any further risk.