Normal Adolescence
By the way adolescence begins around 8 or 9. I have worked with children and adolescents my whole career and have a bachelors in child development.
During this development stage, kids are gaining independence. They are searching for a separate entity from their parents. They need to know they are worthy of life and have a future. They are deciding what their role is in the family, their social groups, and the world.
Their parents are their role models. They begin to decide if they respect their gender counterpart enough to emulate them.
Abnormal Adolescence
Children who have grown up in households with confusing messages learn that they cannot trust themselves, adults, or the world.
They have to survive so they choose a parent to believe. Most children will choose the mother since she is the symbol of nurturing that they desperately need during this confusing time.
They also choose the parent who needs the most help or seems ill. If the father seems like he can take care of himself, they stick with the mother. This gives them purpose.
They are needed in that relationship to hide mom's flaws to the world. Narcissistic women also tell the children they need them and put them in inappropriate roles. Confiding adult information to them. This again makes them feel special and needed.
It is also an easy time to cultivate alienation if the child is in sports or activities that take up their time. Children already hate switching households so if it is made easier to stay with one parent to do activities, and see friends, the children will always choose this. The mother presents visits as an inconvenience rather than a necessary family structure.
To combat this, however inconvenient, you must show up and never give up.