Teancum, seeing that the Lamanite army is heading toward Bountiful, predicts that they will reach the narrow pass. I hypothesize that the narrow pass was a well-known part of an established trade route with the north. Because the Lamanite army had a number of Nephite defectors (their over-king, Amalickiah, being himself a former Nephite), it is probable that this information was readily available to the Lamanite commanders.
Teancum once again moves quickly and successfully to intercept the Lamanite army. The strategic question is why the Lamanite army wants to reach Bountiful and this pass. I hypothesize that the Lamanite army is attempting to cut off Nephite lines of retreat. With the narrow pass in Lamanite hands, the Nephites would not be able to fall back to the north. They were bounded by seas on the east and the west. To the south lay Lamanite lands. They would be surrounded with no easy escape.