Perhaps the teen has a chronic illness that causes diarrhea or other bowel problems. In such cases, diapers can be a practical solution to manage the situation and keep the teen clean and comfortable. It's also possible that there are cultural or family beliefs at play. In some cultures, dealing with such issues in this way might be more acceptable or seen as a way to care for the teen.
It could be related to a developmental delay. If the teen has certain cognitive or physical disabilities that affect their ability to control their bowel movements, diapers might be necessary. Sometimes, it might also be a misunderstanding or miscommunication. Maybe the mom overreacted to a few accidents and thought putting the teen back in diapers was the solution.