Neither the parents nor the teens understand or communicate with each other. Relative to the power and control these teens once had, they feel powerless. Once freedom is given, it isn't easily taken away. Children brought up with the inverted V expend their angry energies trying to recover the power they had too early. Parents do need to take charge from the beginning, and they may indeed need help in learning how to parent.
Competition invades our families.
Although parents usually try to do their best, they may not agree on their values about parenting. One parent may prioritize being a friend to the child as most important. The other parent may believe that setting high standards of discipline is first priority. The kind and caring parent, without recognizing the problem they're causing their children, unintentionally overprotects their children when they face challenge. When children have grown up in an environment where one adult has provided an easy way out for them, they believe they can ignore the parent who expects them to be responsible.