Well, it could be due to a curse. Fanfiction writers like to create new and unique curses. Maybe he's been hit by a curse that affects his nervous system and causes him to pass out. Also, it could be because of physical exhaustion. He's always in the middle of some adventure, and if he overexerts himself, passing out is a possibility.
One possible reason could be a powerful spell backfiring on him. In fanfiction, authors often play around with magic going wrong. Another reason might be extreme emotional distress. For example, if he discovers something truly shocking about his past or his friends.
In fanfiction, Harry Potter might pass out because of a mental attack. Some fanfic authors explore the idea of mind - related magic or dark arts. A strong mental assault on his mind could make him lose consciousness. Additionally, it could be related to some sort of potion side - effect. If he takes an experimental potion, the unexpected consequences might include passing out.
It could also be due to extreme exhaustion. Harry often gets involved in intense battles or has to endure long periods of stress. For example, if he's been fighting multiple foes continuously without a break, his body and magic might give out, leading to him passing out.
One reason could be that he discovers a new threat and decides he can't stay with the Dursleys any longer for their safety. Another might be that he simply can't bear their mistreatment anymore and finds an opportunity to escape.
One possible reason could be that the author wants to create a sense of mystery. By making Harry Potter stop talking, it makes the readers wonder what he is thinking or feeling, and adds an air of enigma to his character. Maybe he has witnessed something so shocking that he is rendered speechless for a while.
The 'Harry Potter' series is far from being a 'criminal fanfiction'. J.K. Rowling created a magical universe filled with positive values. Harry and his friends are constantly fighting against the real criminals in the story, like Voldemort and his Death Eaters. It's a story about good triumphing over evil, not about criminality.
It could be due to some kind of extreme emotional shock. For example, if he discovered something really horrible or had a sudden and intense encounter with an enemy, it might have been too much for him to handle and he passed out.
One possible reason could be that he felt he had learned enough magic to face his enemies on his own. He might have thought that the traditional Hogwarts education was too slow or not relevant to his specific needs, like dealing with Voldemort. Another reason could be some sort of conflict with the Hogwarts faculty. Maybe he didn't agree with their methods or decisions and decided he was better off without their rules.
Maybe it's related to his relationship with Cedric. In the Goblet of Fire, Cedric is a competitor but also a friend in a way. His death is a huge shock in the original story. In fanfiction, it could be that events leading up to Cedric's death or just the general unfairness of the situation makes Harry cry. For example, if there are false accusations against Cedric or if Harry feels he could have done more to prevent the tragedy.
Perhaps he retreats due to the losses he has endured. He has lost many friends and loved ones throughout his journey. This grief could lead him to seek solitude within his own self in fanfiction. Also, he may be trying to come to terms with his identity as both a wizard and a person with a unique destiny. Stepping back into his own self could be a way to reconcile these two aspects.
One reason could be to escape the constant scrutiny and expectations that come with being 'The Boy Who Lived' in Magical Britain. He might feel suffocated by the fame and the never - ending battles against the Dark Arts in his homeland. Another reason could be a quest for new magical knowledge. Magical Britain has its own set of magical teachings, but there could be other, more esoteric forms of magic in other parts of the world that he wants to explore. Also, he might be running away from a personal tragedy or a broken relationship. Maybe a falling - out with his friends or a loss of someone dear to him makes him want to start anew elsewhere.
It could also be related to his self - preservation in the wizarding society. In a world full of magic and danger, showing too much emotion might be seen as a weakness. Harry, being aware of this, decides to shut out his emotions to appear stronger. He has faced a lot of scrutiny and jealousy from some students and even some adults in the wizarding world. By suppressing his emotions, he hopes to be more respected and less of a target.