Another factor could be new magical abilities. Maybe he discovers a power that enhances his learning ability. For instance, he could have a power that allows him to see the knowledge in a person's mind when they explain something to him, which gives him an edge in his studies and helps him get good grades.
One common reason is his determination. Harry has always been a fighter, and in fanfiction, he could transfer that tenacity to his studies. He simply refuses to be bad at academics and works hard to prove himself.
One reason could be that he has a change in motivation. Maybe he realizes that good grades can help him achieve his long - term goals, like becoming an Auror. So he starts to put in more effort into his studies.
It could be for a specific goal, like getting into an advanced magic course. In the fanfiction world, there might be a limited - enrollment class that requires high grades. Harry, being ambitious, would strive to improve his grades to qualify for it.
A common reason is battles. Sasuke is often involved in intense combat situations in fanfiction, whether it's against old foes with new powers or entirely new enemies. His determination to prove himself or protect something often leads him to take risks that result in getting hurt.
Well, in some fanfictions, it could be because of a curse or a jinx that was unknowingly placed on him before he arrived at the Burrow. The Burrow is not always a completely safe place, and with all the comings and goings of wizards and witches, someone with ill intentions could have targeted Harry. Also, he might have eaten something that didn't agree with him. The Weasley family's cooking, while delicious most of the time, could have used some ingredients that Harry's body wasn't used to.
One possible reason could be that he is wrongly accused of using dark magic. In some fanfictions, the Ministry of Magic might be more corrupt or easily influenced, and they could frame Harry for something he didn't do, leading to his expulsion.
One common reason could be a spell gone wrong. Maybe an enemy casts a spell that was intended to weaken him but instead turns him into a child. Another reason might be some sort of magical accident. For example, he messes around with a powerful but unstable magical artifact and it has the unexpected consequence of changing his age.
Equipment failure can be a factor. In fanfiction, his web - shooters could malfunction. Maybe they run out of web fluid at a crucial moment during a fight, leaving him without his main means of movement and defense. As a result, he's more exposed to attacks from villains and gets hurt.
One reason could be jealousy. Maybe Harry has achieved something great like getting a special magical power or a high honor in their magical world, and his friends feel envious. Another reason might be a misunderstanding. For example, Harry could have been wrongly accused of something, and his friends believe the false rumors without giving him a chance to explain.
Well, perhaps in the fanfiction the Hogwarts staff might have been unfair in grading his exams. Like giving him lower marks than he deserved just because they had some sort of prejudice against him. It could also be that they wrongly accused him of something he didn't do, like misbehaving or cheating. Another reason could be that they failed to protect him or his friends from some sort of danger at Hogwarts.
Another possibility is that the fanfiction writer wants to explore a different side of their relationships. By having Hermione and Ron hurt Harry, it shows the complexity of human (or in this case, wizard) emotions. It could be a way to show that even the strongest friendships can have cracks and that mistakes can be made. It also allows for character development as Harry has to deal with this new situation, and Hermione and Ron may later realize their mistakes and try to make amends.