In some fanfictions, it could be related to the magical core inside Harry. Maybe his magical core was damaged in some way, and over time, it weakened to the point where his body couldn't sustain itself anymore. Since sleep is a vulnerable state, that's when his body finally gives in. Also, it could be part of a complex plot where the author wants to explore the reactions of other characters to Harry's sudden and unexpected death while sleeping. It can create a lot of drama and angst among the remaining characters in the story.
Perhaps it has to do with a new and mysterious disease. In fanfiction, new and unique diseases can be created. This disease could be something that attacks his unique chakra system, which is different from others. Since his body is so reliant on chakra, if the disease disrupts it enough, it could lead to his death.
One possible reason could be a powerful and unforeseen dark magic spell within the Chamber. Maybe in the fanfiction, the basilisk had some sort of enhanced venom or an additional power that was able to kill Harry despite his usual heroics.
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.
Perhaps he is betrayed by those he trusts. In the original series, there were instances of betrayal, and if in the fanfiction this is amplified, for example, if the whole wizarding community turns against him unjustly. He might feel so alone and hopeless that he loses his will to live. Also, if he discovers some dark secrets about his past that make him question his own identity and worth, it could lead to this state.
In some fanfictions, Harry might leave his family because of a prophecy or a special mission. He might feel that he has to go it alone in order to fulfill this prophecy or complete the mission successfully. His family might not understand or be able to support him in this, so he decides to leave. Also, there could be jealousy or resentment within the family that makes the environment unbearable for Harry.
One possible reason could be a vampire attack gone wrong. Maybe a rogue vampire targeted Bella and inflicted a near - fatal wound. Another reason might be related to some sort of dark magic or a curse that was unknowingly placed on her. It could also be due to a self - sacrifice situation where she put herself in harm's way to save someone she loves, like Edward or one of her friends.
One possible plotline could be that Harry is suffering from a mysterious curse. Maybe he got it during a battle with a dark wizard. His friends would then be on a desperate quest to find a cure, traveling to far - off places and consulting with various magical experts.
One possible reason could be the overwhelming burden of his destiny. Constantly facing dark forces and the pressure of saving the wizarding world might have worn him down. Another reason could be the loss of his loved ones. For example, if in the fanfiction Hermione or Ron were killed, that could shatter his spirit.
In some fanfiction, Harry might renounce his name to explore a new identity. He has been through so much as Harry Potter, and by giving up his name, he can break free from the predefined roles and destinies associated with it. He could then embark on a journey of self - discovery, free from the constraints of the name that has both defined and haunted him.
One possible reason could be that in the fanfiction, Harry might have discovered some dark secrets about his parents that made him feel betrayed. For example, if he found out they had some hidden and unethical magical experiments in their past.