Magic could be a reason. Like in many fairy tales, a magical spell or a potion can cause the girl to shrink. It could be a curse from an evil witch or a magical accident.
Perhaps it was a side effect of some strange experiment. She might have been exposed to a new type of chemical or radiation in a science lab accident that caused her cells to contract slowly.
It could be a magical or supernatural situation in the story. For example, a spell was cast on her that made her gradually get smaller, and as a result, she shrank out of her clothes. There could also be some sort of scientific experiment gone wrong. If she was exposed to some strange radiation or chemical substance in the story, it might have altered her body size, leading to her shrinking out of her clothes.
Well, in a girl missing story, she might have been a victim of human trafficking. Some criminal gangs target young girls. Or it could be due to a natural disaster. Say, if there was an earthquake or a flood, she could have been separated from her family and be missing. Mental health problems can also be a cause. If she was suffering from depression or some other mental illness, she might have wandered off without realizing.
One reason could be for convenience during travel, like in a long car journey or a flight where access to toilets might be difficult. Another reason might be due to illness or injury. If a girl is bed - ridden or has limited mobility, being diapered can make it easier for her care and keep her clean and comfortable.
In some turned into a girl stories, it could be due to a curse. For example, if someone offended an old witch or a powerful spirit, they might be cursed to change into a girl. Also, there could be a strange natural phenomenon. Like in a place full of strange energy, a person could be transformed into a girl just by being there for a while.
One possible reason could be lack of proper sex education. If a girl is not informed about contraception methods, she may become pregnant accidentally. Another reason might be peer pressure. For example, if her friends are engaging in early sexual activities without thinking of the consequences, she may follow suit. Also, in some cases, it could be due to coercion or abuse. If a girl is forced into sexual relations, she may end up pregnant.
Sometimes, it's a result of a planned diet change. Maybe the girl realizes she's too thin and wants to have a more curvaceous body. So, she starts consuming more calorie - dense foods like cheese, chocolate, and fatty meats. Or perhaps she's an athlete who has been training hard and now needs to gain weight for better performance. In this case, she'll focus on eating more protein - rich and high - calorie foods, which will lead to her fattening up in a healthy way.
Jealousy can be a reason. Just like in the story I mentioned before where two girls liked the same boy. Also, misunderstandings. For example, if one girl thought the other was talking bad about her behind her back when it was not true at all. Another reason could be competition, like competing for a role in a school play or a spot on a sports team.
Sometimes, it could be due to a lack of communication in the relationship. If she can't talk to the boy about her feelings or problems, she might look for someone else to confide in. Also, peer pressure or the influence of her friends can play a role. If her friends encourage her to be with someone else or make her feel like she can do better, she may be more likely to cheat.
The most common reason is the way she dresses. If a girl wears masculine - style clothes like baggy pants, oversized shirts and has short hair, she can be easily mistaken for a guy.