What is the psychological reason for kids who hold in their bowel movements?

Control. Children often feel like they have little or no control over their environment, and stooling is something they can sometimes control. It's the one battle of wills that kids can win over parents. Other times, it's about avoidance, of either pain or the sensation of loss (yes, they are attached to the poop sometimes). One good hard bm and they will try to not have another one.
Depends on cause. Many conditions cause encopresis. The most common cause in normal kids is adhd. Poor habits, diet, fluid intake, fear are the other causes. Kids with spina bifida or other neurological diseases have difficulty evacuating. Rarely, hirschsprung's disease or another rare intestinal problem is the cause .