Verse 21 confirms that the Nephites would sortie from their defensive position to harass the Gadiantons. The combination of the effects of those losses, and the problem of food, has Zemnarihah try a different tactic. In this case, he attempts to go north. That is not explained, and somewhat unusual, because the Gadianton stronghold was in the south.
As speculation, if the Gadiantons did not have the connections to the northern city and culture, then this attempt to go north was to head toward safety, back to their homelands.