Maybe she is in a marshy area. Marshes are often soft and muddy. If she steps in a particularly muddy part, she will start to sink. The fact that she is barefoot means her feet can penetrate the mud more deeply compared to if she had shoes on. Also, she might be too light - weighted and not be able to resist the downward pull of the soft ground.
A barefoot girl was strolling on the beach at sunset. The sand near the water was wet and soft. As she walked further towards the sea, she stepped into a spot where the sand was like quicksand. She started slowly sinking. She thought about how she got into this situation and realized she should have been more aware of her surroundings. She then wiggled her feet and used her hands to push herself up bit by bit until she was free.
It's possible that she had some kind of accident with her shoes. For instance, the heel broke or the strap snapped, and instead of going home to change, she decided to continue shopping barefoot.
She could be barefoot because she was playing a game like hide - and - seek outdoors and got stuck in a bush while trying to hide.
She could have been a victim of a prank. Some naughty kids might have hidden her shoes as a joke while she was sitting in a public square, leaving her barefoot and stuck there.
One reason could be that her feet are more sensitive because she is often barefoot. The soles of the feet have a lot of nerve endings, and without the protection of shoes, they might become more sensitive over time. So, even a light touch can make her desperately ticklish.
Maybe she's in a really good mood and wants to have some extra fun. Tickling can be a form of affectionate play, and she might be craving that kind of interaction.
Maybe she stepped on something sharp like a stone or a thorn. It could be really painful and that's why she regretted it.
She might be embarrassed. Ticklishness can seem like a silly or vulnerable thing, so she doesn't want others to see her react in a way she might consider uncool.
One possible reason could be poverty. If the girl comes from a very poor family, she might not have proper footwear. Another reason could be some form of punishment. Maybe she did something wrong according to the rules of a particular institution or group. However, this is really cruel and unfair treatment for a girl.
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.