It could be due to some sort of time - related accident. For example, if Danny was messing around with a time - traveling device or a ghost power that had something to do with time, and it malfunctioned. This would create a situation where his physical form regressed to that of a baby, and he'd have to deal with not only being a baby but also the consequences of the time distortion.
Perhaps a new type of ectoplasmic energy affected him. There might be a strange new form of ectoplasm in the Ghost Zone that Danny encounters. His body, being half - ghost, has a unique reaction to it and turns into a baby. His friends and family would then have to search for a way to counter this new energy to get Danny back to normal.
One possible reason could be a spell gone wrong. Maybe an enemy cast a spell aimed at weakening Danny, but it backfired and instead turned him into a baby. This would add an interesting twist as Danny would have to rely on his friends to protect him while they figure out a way to reverse the spell.
One possible reason could be that he's pushed too far by the constant attacks from ghosts. Maybe he gets tired of always being the good guy and having to protect everyone all the time. It could be that he feels unappreciated and that could lead to a dark turn.
In the fanfiction, it could be due to an experiment. Some mad scientist - like character in the Naruto universe might have been experimenting with a new form of chakra manipulation. Gaara, being in the wrong place at the wrong time, got caught in the experiment and was turned into a baby. This would then lead to a whole new set of adventures as his friends try to find a way to reverse the process.
Well, it might be a form of catharsis for some writers. They may have their own experiences or feelings of pain and suffering that they project onto the characters through torture in the fanfiction. Another reason could be that they are simply trying to break the boundaries of the normal 'Danny Phantom' narrative. By introducing such a dark and disturbing element like torture, they are trying to create a new kind of story that challenges the traditional perception of the show. However, this does not mean it is a widely accepted or positive form of fanfiction, but it exists due to these complex reasons.
It might be that the people in Amity Park don't fully understand him. They might be scared of him or misjudge his actions, which could lead to Danny hating the place. For instance, if they blame him for some damages that were actually caused by the ghosts he was fighting.
Obviously, Danny Phantom himself could be the 'baby daddy'. His dual nature as a human and a ghost could bring a lot of interesting twists to the story of having a baby. For instance, the baby might inherit some of his ghost powers, which would create all kinds of challenges for raising the child.
In the fanfiction, it could be that Danny's friends have their own problems to deal with. They might think that being associated with Danny is bringing more trouble into their lives. Maybe they get threatened by a new villain who warns them to stay away from Danny. Or they could simply be growing apart as they enter different phases of their lives and their interests no longer align with Danny's.
One possible plot could be that Danny and Sam face challenges in raising their baby while also dealing with Danny's ghost - fighting duties. Maybe the baby inherits some of Danny's powers and they have to figure out how to help the baby control them.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
One possible reason could be that she's lost in an unfamiliar place. In the Naruto universe, it's a dangerous world for a baby. Maybe she got separated from her parents during an attack on the village. Another reason might be that she's hungry or not feeling well. Just like any baby, she could be crying because she needs something basic like food or warmth.
Well, perhaps in the fanfiction, Canada turns into a baby due to some magical mishap. The nations have to search for a way to turn him back. Meanwhile, baby Canada is just exploring the world around him innocently. He might crawl over to England's tea set and accidentally break a cup, leading to England's typical over - reaction.