Here the text stresses the great purity of Job's children. Since [Job] did not perceive any sin in them, he sacrificed because of [the sons'] disposition. Job was aware that the human weakness and sluggishness that mark young persons often escalates. This is also why St. Paul said, "I am not aware of anything against myself." And the psalmist, "Forgive my hidden faults." Therefore we have to believe that the sons of Job did not die because of their own sins.
Source: COMMENTARY ON JOB 1:5