The Lord releases them from prison, and the walls of their prison break open so that they may leave.
Literary: This story is told as a moral story. The bad appears to have the good in a desperate situation. The good are miraculously saved, and the bad are killed. This formula of vindication is an ancient one, and one with which human beings continue to resonate. This does not suggest that the events did not take place precisely as stated, but rather that the way Mormon couches them, and indeed the reason for telling this story in the first place, have to do with those feelings of vindication. The prevalence of this theme in all of the oral literature of the world, where the oppressed with whom the teller sympathizes is miraculously rescued and the wicked punished is such that it may be comfortably stated as a nearly universal human concern. Mormon was not immune from his humanity, and therefore not only chooses this story, but couches it in those unmistakable terms.
Geographic: The events noted here suggest that the means the Lord used to effect the escape of Alma and Amulek was a particularly violent earthquake. We are particularly told that the "earth shook mightily." This shaking of the earth opened the breach in the walls, and the collapse of the roof and or walls easily supplies the means whereby the lawyers and the chief judge perished. In the area of the world where these events are suggested to have taken place, earthquakes are well known and frequent. Indeed the area is sufficiently well known for earthquakes that Eric R. Wolf would name a novel about that area of the world Sons of the Shaking Earth.