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.
Edward would likely be in a state of panic. He loves Bella deeply, so he'd probably do everything in his power to save her. He might search for a cure or a way to reverse whatever is causing her to die.
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.
One possible reason could be a curse that was placed on her secretly. Maybe an enemy from her past, someone she thought was defeated, managed to put a slow - acting curse on her. Another reason might be a side - effect of some powerful magic she used to save someone or solve a difficult problem. For example, if she used time - traveling magic in a wrong way, it could have negative consequences on her health.
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 curse placed on him. Maybe an enemy with dark magic managed to put a slow - acting curse that is gradually sapping his life force.
One possible reason could be a powerful enemy attack. Maybe she was caught in a fight against a much stronger foe who used a deadly jutsu on her. Another reason might be a self - sacrifice. Sakura could be in a situation where she has to give up her life to save her teammates from Team 7.
One possible reason could be that she discovers some dark secrets about the Cullens. Maybe she finds out they have been lying to her about their true nature in a more malicious way than in the original story. Another reason might be that she feels threatened by their vampire powers. If they accidentally or purposefully use their powers around her in a menacing way, it could make her feel scared and turn that fear into hatred.
One example could be a story where Twilight Sparkle sacrifices herself to save Equestria from a powerful dark magic. Maybe she uses all her magic energy to create a shield that stops an evil force but in the process, it drains her life completely. Another could be a story of an incurable illness in Equestria that Twilight contracts and despite the efforts of her friends, she succumbs to it.