Geographical: Sorenson notes that the information that Gideon gives is too scanty to trace a certain route out of Lehi-Nephi. However, he does suggest a plausible route that bends toward Shilom on the south before it arrives at a more passable valley system leading northward to Zarahemla (Sorenson, John L. An Ancient American Setting for the Book of Mormon. FARMS 1985, p. 175).
The evidence of the relative locations of Shemlon, Shilom, and Lehi-Nephi suggests that Shilom lay in between Shemlon and Lehi-Nephi. At this point in time, all of Limhi's people are located in Lehi-Nephi. This would indicate that either Shilom is uninhabited, or that it is inhabited by Lamanites. This latter is more probable, as the Lamanites appear to be in an expansion mode, and Noah had built buildings there that would be of value to the Lamanites.
With this configuration in mind, the "back gate" most likely would be the one farthest from Shemlon and Shilom, allowing the people to leave with the city between them and the largest force of the Lamanites, since they would not want to volunteer to go too close to Shemlon if they could avoid it.
In the geography Sorenson has developed, their path would proceed down a valley to the south-southwest before they hit the northwest valley. This saved them from crossing mountains with their people and herds. It might have been safer to cross over the mountains, but much more difficult on women, children, and flocks. Thus this turn towards Shilom makes geographic sense, even when it makes no sense from a safety perspective.