With the battle ended, those who had remained at Cumeni were finally able to ask about the reason that the men who had guarded the prisoners had been able to return so quickly. Since those men were some of those credited with saving the Nephites, the questions were for information only, and not for suspicion.
What was discovered was that the guards had learned of the Lamanite attack, and the Lamanites being guarded also heard of it. That gave the Lamanite prisoners the courage to attack, which they did.