The Amlicites met a large Lamanite army, and joined with them. Of course, by the fact of rebellion, they had become enemies of the Nephites, and, therefore, considered Lamanites. Reading between the lines of the story, it is possible that this joining of forces was part of the plan. Perhaps, had the Amlicites been successful, the Lamanites would have invited them and formed a new alliance. In this case, the Amlicites are fleeing and meet with the Lamanites.
It is that fact itself that suggests that there may have been some prior understanding. It is difficult to imagine an army in enemy territory passively waiting to accept a large number of armed men running quickly towards them. It would surely have been bloodshed first and questions later. If, however, the meeting had been arranged, it would explain why the Amlicites survived the encounter.
The new army was greater than the force that Alma led, so the Nephites retreated towards Zarahemla, perhaps as a more defensible position, perhaps as the location for reinforcements.