Perhaps he has discovered a great threat that is specifically targeting him. To keep the village and his family from being collateral damage, he fakes his death. This way, he can secretly gather information, train more, and come back stronger to face the threat when the time is right.
One possible reason could be to protect his family. After the war, Naruto might have made many enemies. By faking his death, he could draw the attention of these enemies away from his family, especially his wife and children. His enemies would think he's no longer a threat and stop targeting his loved ones.
Well, Naruto could have made a deal with some powerful being or used some sort of forbidden technique. Maybe he found a way to transfer his life signs to another object or being, making it seem like he was dead. He's always been resourceful and full of surprises, so faking his own death in a really elaborate and unexpected way is not out of the question.
It could be Orochimaru. Even though he's had his moments of redemption, he might relapse into his old ways. He could see Naruto's 'death' as an opportunity to carry out some evil plan that Naruto would have stopped. Maybe he wants to use Naruto's death to create chaos and then take advantage of the power vacuum in the ninja world.
One possible plotline could be a powerful new enemy emerging that neither Naruto nor Boruto can defeat alone. Maybe an ancient evil that has been awakened and has powers far beyond what the ninja world has ever faced. Their deaths could be a sacrifice to seal this evil away, protecting the village and the world at large.
Well, he might use some substitution jutsu. He could replace himself with a log or something similar at the crucial moment, making it seem like he's been killed.
His best friends Hermione and Ron would be the most likely candidates. Hermione's intelligence and vast knowledge of magic could come up with a complex plan to fake Harry's death. Ron, with his loyalty and bravery, would be there to support and execute the plan. They have been through so much together that they would be willing to take such risks for Harry.
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.
In the 'boruto kills naruto fanfiction', a motive could be a power struggle. Boruto might believe that Naruto's power is holding back the village's growth or his own potential. He could think that by eliminating Naruto, he can bring about a new era of power in the Hidden Leaf. Also, it could be out of desperation. If Boruto is in a situation where he believes that killing Naruto is the only way to save someone he loves, like his mother or a friend, he might take such a drastic step.
Maybe it's the high expectations that Naruto has for Boruto. Naruto is a hero, and he might unknowingly put a lot of pressure on Boruto to follow in his footsteps. Boruto could be scared of not being able to meet those expectations.
Maybe she's jealous of Naruto's popularity or his strength. It's possible that she feels overshadowed by him all the time and that jealousy turns into resentment, leading her to hurt Naruto.