Perhaps it's related to his family history. He grew up without his parents, and there could be some underlying emotional issues because of that. The lack of a normal family upbringing might make him uncomfortable with physical contact. And in the wizarding world, there are many dangerous spells and curses that could be transferred through touch, which could also contribute to his fear.
It could be due to his sense of personal space. Harry has been through a lot, and he might have developed a strong need to keep a distance from others as a form of self - protection. Also, his fame as 'The Boy Who Lived' could play a part. He might be constantly worried about people touching him out of some ulterior motives, like trying to use him for their own gain.
One possible reason could be past trauma. Maybe he had a bad experience in his childhood, like being bullied or physically hurt by someone. Another reason might be related to his magical abilities. He could be afraid that being touched might somehow disrupt his magic or cause an unexpected magical reaction.
One possible reason could be his past traumas. Harry has faced many dangerous and life - threatening situations. For example, the numerous attacks from Voldemort and his Death Eaters. These experiences might have made him overly cautious and averse to physical touch as it could potentially be from an enemy. Also, his upbringing in the Dursley's household wasn't exactly warm and fuzzy. The lack of positive physical contact there could have contributed to this fear.
Perhaps his uncle has a very intimidating appearance or a loud voice that scares Harry. In the fanfiction, it could be that his uncle has a certain look in his eyes when he's angry that makes Harry tremble with fear.
One possible reason could be his past experiences with some cruel adults, like the Dursleys. They treated him horribly, so in fanfiction, this trauma might make him generally fearful of adults. Another reason could be his experiences with certain authority figures at Hogwarts who may have been strict or unfair, which could be exaggerated in fanfiction to create a fear of adults.
One possible reason could be that they had a past traumatic experience related to explosions. Maybe they witnessed an explosion that caused harm to their loved ones or themselves. Another reason might be that explosions in the 'Naruto' world are often associated with powerful and dangerous jutsus or attacks, and they are simply afraid of the destruction and chaos that explosions can bring.
Another way is to explore the backstory of his fear more deeply. Through flashbacks, the author can show the readers exactly what events led to his fear. This can create more empathy for Harry. And then, the story could progress as Harry starts to face his fear when he has to, like when he needs to be healed by a healer's touch.
One reason could be a past bad experience. Maybe he had a really painful treatment as a kid that left a deep mark. For example, if he had a serious injury once and the doctor was rough while treating him, it would make him fear doctors.
One possible reason could be Lockhart's over - the - top self - promotion. In the original story, Lockhart is constantly boasting about his achievements which are actually false. In fanfiction, this could be exaggerated, making Harry see him as an untrustworthy and annoying figure, thus causing fear. Another reason might be that Lockhart could be portrayed as having some hidden and sinister motives in fanfiction, which Harry senses and is afraid of.
One reason could be that the official Harry Potter canon is so extensive and detailed that it overshadows fanfiction. The original story has such a rich world, characters, and plotlines that it becomes the main focus for many fans, leaving less room for fan - created stories.
One reason could be extreme stress. In fanfiction, Harry might be facing a new and overwhelming threat, like a powerful dark wizard that even his skills can't easily handle. The pressure of having to protect his friends and the wizarding world could cause him to faint.