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.
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.
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.
One possible reason could be extreme loss. If Cal were to lose all his friends and allies in a tragic way, he might feel so much pain and anger that it drives him towards the dark side. For example, if his newfound Jedi family was wiped out by the Empire, the grief could cloud his judgment and make him vulnerable to the dark side's temptations.
In some stories, it could be due to a magic spell. Just like in fairy tales, a wicked witch or an inexperienced wizard might accidentally cast a spell that changes a person's gender. Another reason could be a scientific experiment. Scientists might be trying to do something revolutionary like gender transformation through some new technology, but things go wrong and a male turns into a girl.
It might be that the man made a deal with a witch and broke the terms, causing the transformation. Or it could be a symbol of his inner transformation and represented metaphorically as a physical change into a dog. There could also be a virus or a strange potion involved.
One reason could be extreme stress. In a story, a character might face so much pressure from work, relationships, or other aspects of life that her mind unconsciously reverts to a more innocent and carefree state like that of a baby girl. Another reason could be a magical or supernatural event. For example, a spell or a strange encounter with a magical being could cause the age regression.
One reason could be friendship. Gaara and Naruto have a special connection in the original story. In fanfiction, this connection could be deepened, and Gaara would save Naruto because he values their friendship. Just like Naruto once reached out to Gaara, Gaara would do the same for Naruto when he's in danger.
In a tf story, it could be due to a scientific experiment. Scientists might be trying to find a way to give humans the abilities of Pokémon. The girls could be test subjects, and something unexpected happened during the process. Also, it could be related to a curse. If they entered a forbidden area or angered a powerful Pokémon - like spirit, they could be cursed to turn into Pokémon. This would add an element of mystery and danger to the story.