Distress from Satan
Indeed, those who turned away on the day the two armies met were made to slip by Satan due to their past deeds. But Allah has forgiven them.
3:155Lo! those of you who turned back on the day when the two hosts met, Satan alone it was who caused them to backslide, because of some of that which they have earned. Now Allah hath forgiven them. Lo! Allah is Forgiving, Clement.