In some fanfictions, it could be due to a magic spell gone wrong. Maybe a wizard or a witch was trying to do something else and accidentally turned Hiccup into a dragon. Another reason could be a cursed object. Hiccup might touch or use an object that is cursed and that causes the transformation.
It could be a result of a dragon's special ability. There might be a very powerful dragon in the fanfiction that has the power to transform humans into dragons. Maybe Hiccup gets too close to this dragon during a battle or while trying to study it, and the dragon uses its power on him. Also, some fanfictions might have it as a form of punishment from the dragon gods for Hiccup breaking some kind of ancient rule.
One reason could be a jutsu gone wrong. Maybe an enemy uses a forbidden jutsu on Sasuke that has an unexpected side effect of changing his gender.
There's a fanfic where Hiccup is turned into a dragon by a mysterious spell. The story focuses on his journey to find a way to turn back. Along the way, he discovers new dragon abilities that he didn't know existed. It's a great story that combines elements of mystery and adventure. The author does a wonderful job of describing Hiccup's thoughts and feelings as a dragon.
One possible theme is friendship. The relationship between Hiccup and his dragon is a strong form of friendship. They rely on each other, trust each other, and grow together.
One common reason could be a spell gone wrong. Maybe an enemy casts a spell that was intended to weaken him but instead turns him into a child. Another reason might be some sort of magical accident. For example, he messes around with a powerful but unstable magical artifact and it has the unexpected consequence of changing his age.
A possible motive could be revenge. If a dragon had hurt someone he loved, like his father or Astrid, he might be driven to kill it. It would be a very emotional and difficult decision for him, but his love for his friends and family could push him to that extreme.
One possible theme could be growth and self - discovery. As a hatchling, Hiccup would be learning about himself and his place in the dragon world.
One reason could be revenge. Maybe someone he wronged in the past, like a fallen angel or a demon he betrayed, decides to take their vengeance by stabbing him. It adds a layer of drama to the story.
One possible reason could be that he breaks some important rules within the fan - created world. For example, if the fanfiction has a strict code of honor and Naruto is made to cheat in a crucial competition, that could lead to his disqualification. Another reason might be that he is framed by an antagonist. The antagonist could plant false evidence against Naruto, making it seem like he has violated the rules, thus getting him disqualified.
One possible reason could be that she's different. In many fictional worlds, being unique can make a character a target. Maybe her appearance or her abilities set her apart in a way that makes others jealous or feel threatened.
Hiccup's over - confidence could also make Astrid mad. If he goes into a dangerous situation without proper preparation, thinking he can handle it because of his previous experiences, and then things go wrong. Astrid, who believes in careful planning, would not be happy with his reckless behavior.