In fanfiction, Harry could faint due to some sort of magical disease or curse. There could be a new, made - up illness that affects wizards and one of its symptoms is fainting. Or perhaps he's in a situation where he's cut off from his magic for a while and his body, which is so used to having magic, goes into shock and he faints. It could also be that he's in a place with really strange or strong magical energy that his body can't handle, and this causes him to pass out.
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.
It could be due to extreme exhaustion. Harry has been through a lot in his adventures. He might have been overusing his magic, perhaps in some secret training or in trying to protect Hogwarts from an unseen threat, and his body just couldn't take it anymore, causing him to faint right there in the Great Hall.
It could be due to some kind of self - sacrifice he made in a previous battle. For example, he might have used a very dangerous spell that had a side - effect of slowly killing him to save his friends or the wizarding world.
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.
Poison or some sort of harmful jutsu effect. Maybe an enemy uses a special poison or a jutsu that slowly weakens Naruto's body. In the fanfic, this could be a new and unique type of attack that he has not encountered before, and his body is not able to withstand it, resulting in fainting.
One reason could be to explore the darker side of human nature. Even though Harry is a hero in the original series, fanfiction writers might want to see how he would be affected if he was pushed to his limits. Another reason is to create a more complex and controversial storyline. It grabs readers' attention as it goes against the normal perception of Harry.
Some fanfiction writers may have a different vision for Harry Potter's character. They might see him as more of a diplomat or a healer within the wizarding world, rather than a fighter. So, they write stories that reflect this different view of the character, where he uses his wits and other skills rather than engaging in physical or magical combat.
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 safety. Some fanfiction authors might think that Hogwarts is too dangerous for Harry, so they make him homeschooled to keep him out of harm's way.
One reason could be the extreme hardships the characters face in the original story. For example, Harry has lost his parents, faced Voldemort numerous times, and seen many friends die. In fanfiction, this could be amplified to a point where a character feels they have no will to live. Another aspect could be related to the sense of isolation. Some characters, like Snape, were always sort of alone in their struggles, and in fanfiction, this could lead to a loss of the will to live.