One possible reason could be that the Kyuubi's power goes out of control completely. Since Naruto has the Kyuubi inside him, if he fails to maintain the balance, the Kyuubi's raw and unrestrained power might end up killing him.
One possible consequence could be chaos in the ninja world. With Naruto, a major force for peace, dead and the Kyuubi released, other villages might see an opportunity to gain power or territory. Villages that once relied on Naruto's protection would be vulnerable. Also, the Kyuubi, being a powerful and wild force, could start causing destruction on its own, without Naruto to control it.
It could be a way for fans to further explore the deep connection between Naruto and the Kyuubi. Since their bond is so strong in the original story, some might take it to an extreme with the'mate' concept, thinking about how their relationship could be on a more intimate level in a different, non - canon way.
Some fanfictions might explore the idea of Naruto being betrayed by someone close to him. This betrayal could lead to a situation where he is caught off - guard and killed. It adds a dark and dramatic twist to the story.
One reason could be a misinterpretation of the Naruto - Kyuubi relationship. If a writer doesn't fully understand the journey of Naruto and Kyuubi towards mutual respect and cooperation in the original story, they might come up with this idea. Another possibility is that they are trying to create a new and edgy storyline that caters to a niche audience who likes to see extreme power imbalances in fanfiction.
One possible reason could be the loss of a dear friend. Naruto values his friends deeply, and if he were to lose one, it would break his heart. For example, if Sasuke were to die or turn against him completely in a way that Naruto couldn't fix, it would be a huge blow.
Perhaps there was a curse or a disease that he couldn't overcome. In some fanfictions, they like to introduce unique and deadly diseases that not even the best medical ninja can cure. It could also be due to a failed mission where he was caught in a trap that was impossible to escape from, leading to his death.
It might also be due to a sense of hopelessness. If he's faced with an enemy that seems truly unbeatable and he fears the destruction of the village or his friends, he could ask to be killed to prevent further harm. For example, if there's a powerful villain who has Naruto's friends hostage and the only way to save them is by Naruto sacrificing himself.
One reason could be to explore the darker side of the Naruto universe. Writers might want to see how the characters and the world would react to such a major loss.
One possible reason could be that she sacrifices herself to save Naruto from a powerful enemy. In the heat of battle, she might put herself in harm's way to ensure Naruto's survival.
There might be some 'naruto is killed by sakura fanfiction' out there. One possible reason could be to explore a dark and alternative storyline. Maybe in that fanfiction, Sakura has been corrupted or misdirected in some way, perhaps by an evil force that makes her turn on Naruto. Another reason could be to create a high - impact, emotional story where the death of a main character like Naruto at the hands of someone close shakes up the entire Naruto -verse in that fan - created narrative.