Well, perhaps in the fanfiction world, Naruto's parents discover some kind of 'dark power' within him that they fear might be uncontrollable. They think that banishing him is the only way to keep the village safe from this potential threat. This could be a power that is different from the Nine - Tails' chakra and seems extremely dangerous to them.
One possible reason could be that Naruto discovers some dark family secret that his parents want to keep hidden. For example, if there was a connection between the Uzumaki clan and some forbidden jutsu, and they fear Naruto might be exploited because of it. So they banish him to protect the clan's reputation and his own safety.
Well, in fanfiction, his family might banish Naruto due to differences in ideology. Naruto could be advocating for a more inclusive and progressive approach within the family, which goes against the traditional and conservative values of his family. They may see his ideas as a threat to the family's long - established order. Also, if there is a prophecy or some sort of family superstition related to Naruto that they misinterpret, they might banish him to avoid what they think could be a disaster for the family.
One possible reason could be false accusations of a serious crime. For example, if he was wrongly blamed for a major attack on the village. Another reason might be political intrigue. Some powerful figures in the village might see Naruto as a threat to their power and find a way to banish him to maintain their control.
Maybe in the fanfiction, there were some political motives. Hiruzen might have been pressured by other hidden villages. They could have threatened Konoha if Naruto, as the Nine - Tails' jinchuriki, remained in the village. So, to avoid a potential war or conflict, Hiruzen made the difficult decision to banish Naruto. Also, it could be due to a misunderstanding. Naruto might have been in the wrong place at the wrong time during an incident, and Hiruzen misjudged the situation.
One possible reason could be for the protection of the village in a very extreme and controversial way. Minato might think that Naruto's presence is attracting too much danger and decides to banish him to keep the village safe.
One possible plotline could be that Naruto is banished due to false accusations of endangering the village, despite his parents being alive. His parents, Minato and Kushina, might be powerless to stop the decision because of some political intrigue within the village. Naruto then has to find his own way in the outside world, facing new enemies and making unexpected allies.
Another reason could be that Naruto is lured away by an external force. Some powerful figure or group might convince him that leaving his parents is for the greater good, perhaps in the name of protecting them or the village in a more effective way.
In fanfiction, it could be due to a prophecy. Suppose there was a prophecy that Naruto would bring great danger to the village in the future. Tsunade, despite her bond with Naruto, might believe in the prophecy and banish him to prevent the foreseen disaster. Also, it could be related to Naruto's power. If his power was growing too rapidly and becoming uncontrollable, Tsunade might banish him to protect the village from any potential harm his power could cause.
One possible reason could be political pressure. Maybe some powerful clans in the village see Naruto as a threat due to his growing power and influence. They might force Tsunade's hand to banish him to maintain their own status quo in the village.
One possible reason could be fear. The villagers have always feared the Nine - Tails within Naruto. His family might banish him out of this same fear, not wanting to be associated with the potential danger he represents. Maybe they think that by banishing him, they can keep the village safe from the Nine - Tails' power getting out of control.