In the fanfiction, it might be that there was a powerful and unexpected dark magic curse placed on the Potter family long before Voldemort's attack. This curse was designed to target the youngest Potter, and despite the efforts of his parents, it couldn't be stopped, causing Harry to die as a baby.
Maybe he was killed by Voldemort in a more sudden and unexpected way when he was a child. Since Voldemort was always after him, it's possible that he managed to get to Harry earlier in this fanfiction.
Maybe he was in a dangerous duel with a powerful dark wizard. The dark wizard could have cast a spell that was specifically designed to maim or disarm in the most literal sense.
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.
In fanfiction, it could be related to a magical accident. Maybe a spell went wrong and changed the color of his hair, and then he decided to keep it and even dye it a different color on purpose. It could also be a part of a transformation arc, where changing his appearance, starting with his hair, is a symbol of his growth and change as a character.
Well, it could be that Harry Potter makes a big mistake in fanfiction. For instance, he accidentally endangers his mates. This would lead to them feeling angry and rejecting him. Also, differences in beliefs or goals can play a part. If Harry starts to pursue a path that his mates don't agree with, like getting too involved with a certain type of magic that they consider dangerous, they might reject him.
In some fanfictions, his family might be involved in dark magic or have a bad reputation. So, Harry Potter refuses them to uphold his own moral values. He is known for his strong sense of right and wrong, and if his family goes against that, he would distance himself.
One possible reason could be that he might feel abandoned. Growing up without them, he could resent the fact that they left him an orphan. He might think that if they were really so great, they would have found a way to survive and be there for him.
Perhaps Harry faked his death with the help of his friends. Hermione, being extremely intelligent, could have come up with a plan. They might have used polyjuice potion to create a false body that looked like Harry's dead body. This would mislead the enemy and give Harry the element of surprise in the ongoing battle against the dark forces.