Geography: The geography of the this event is critical because we have to explain how it is that the Amlicites could have gone farther than the Nephites, and yet be in a position to return to Zarahemla as fast as, or potentially faster, than the Nephites who had not traveled as far.
Using Sorenson's map of the topography of the proposed area, the Amlicites have a rough crossing over a mountainous region, and then a crossing of the Sidon. When the Nephites gain word of the Lamanite army, they travel directly over the mountains to a ford in the river Sidon. Both the Amlicites and the Nephites likely took advantage of breaks in the mountain range rather than climb to the tops of the mountains. (See Sorenson 1985, p. 154 Map 7).
Social/Historical: The are several aspects of this particular event that are noteworthy. The first is that the Amlicites joined with the Lamanites. This is given as a quick statement, and almost as though this would be such a simple thing to do. We must remember that the Amlicites were fleeing from a battle. Thus they would be dressed for battle, armed for battle, and rushing. While it is true that they were rushing in retreat, they were still approaching the Lamanite position rapidly in full battle regalia and weaponry. That very fact would have necessarily caused some immediate consternation and defensiveness on the part of the Lamanites.
The Nephite spies do not describe this initial meeting of the Amlicites and Lamanites, but it surely began with some tense moments that required some rapid reconciliation.
It is also obvious that there was some recognized mechanism for joining with the Lamanites. The priests of Noah were able to do it quite easily, and the Amlicites appear to have been able to do so with equal ease, despite being Nephites and dressed for war. The ethnic identity of the Amlicites apparently held no inherent impediment for joining with the Nephites. We may freely surmise that there was some type of lingua franca available to the two peoples. The circumstances of this particular meeting would fairly necessitate full mutually intelligible communication to overcome the militaristic appearance of the Amlicites.
On the other hand, Sorenson suggests that this meeting was planned beforehand, and that the Amlicite attack on the East side of the Sidon was calculated to draw the Nephite army off away from Zarahemla, and thus leave it more vulnerable from the main Lamanite attack on the West side of the Sidon (Sorenson 1985, p. 196).
While there is no direct hint at collusion between the Amlicites and the Lamanites in the text itself, the particulars of the Amlicite attack make this scenario quite plausible, and perhaps best explains how the Amlicites could come upon the Lamanite army in full battle regalia and be accepted so quickly. If they were already allies and the retreat to the Lamanite army were part of the plan, then the explanation is simple.
This plan would have had two contingencies. The first would be that the Nephites curtailed pursuit (which was what happened). The second contingency would be if the Nephites continued their pursuit of the Amlicites. This would have changed the location of the battle, but the Lamanites could still have engaged a Nephite army that was not necessarily prepared for a full battle formation of the Lamanite army.
The next important aspect of this meeting was that the Lamanites were there at all. The meeting takes place in the "course of the land of Nephi." This appears to indicate that they Lamanites have come up from the Land of Nephi. This is first clear indication of Lamanites from the Southern lands coming to battle in the land of Zarahemla. While there have been other skirmishes with Lamanites, none of those tell us where the Lamanites came from. When we remember that the physical connection between the lands of Nephi and Zarahemla was difficult, and the Book of Mormon records various groups getting lost moving through it (Limhi's expedition attempting to find Zarahemla misses the land of Zarahemla entirely, and the Lamanite army following Limhi's people gets lost) then we can appreciate the significance of this large Lamanite army appearing at this particular location.
This very difficulty of finding Zarahemla from the more Southerly Lamanite lands further strengthens the suggestion that the Amlicites were in collusion with the Lamanites. As Sorenson noted above, Chiapa de Corzo had connections to the area that the Book of Mormon would consider the Lamanite lands (Sorenson, 1985 p. 197). They could have exploited those connections to create this plan for the conquest of Zarahemla.