Maybe Harry accidentally frames Weasley for something. It could be due to a misunderstanding, like a spell gone wrong that makes it seem like Weasley was involved in some illegal magic use.
Maybe it's a corrupt Ministry of Magic official. They might use Harry's credibility to their advantage and trick him into providing false information about Weasley. They could be after the Weasley family for political reasons or because they want to cover up their own misdeeds by creating a distraction with Weasley's arrest.
One possible plot could be that evidence of her illegal use of power and abuse in Hogwarts is finally uncovered. Maybe a student bravely comes forward with proof of her cruel punishments and false accusations. This leads to the Ministry of Magic having no choice but to arrest her.
One possible plot could be that mankae is wrongly accused of a crime. Maybe he was at the wrong place at the wrong time. For example, he was near a store that got robbed and due to some misunderstandings, the police think he was involved. So he gets arrested and has to prove his innocence throughout the fanfiction.
One possible plot could be that Dumbledore is framed for a crime he didn't commit. Maybe there's a dark wizard who has managed to plant false evidence against him. For example, evidence that he was involved in some sort of dark magic plot. Another could be that the Ministry of Magic, fearing his power and influence, decides to arrest him on false pretenses to gain more control over Hogwarts. This could lead to a lot of chaos in the wizarding world as Dumbledore has many allies and supporters who would not stand for his unjust arrest.
Maybe Ryoga got arrested for being in the wrong place at the wrong time. For example, he might have been near a crime scene just as the police arrived and due to some misunderstandings, like his strange appearance or his confused state which made him act suspiciously, he was taken into custody.
In a Tsuna gets arrested fanfiction, it could be that a new enemy discovers a loophole in the law and uses it to have Tsuna arrested. This enemy might be from a rival family or an unknown organization. Tsuna's arrest would then be a way to start a new arc in the story. His friends like Gokudera and Yamamoto would likely be very determined to get him out. They could start their own investigation, and in the process, uncover secrets about the enemy and the legal system in their world. It could also lead to Tsuna making new allies within the prison or among the legal authorities who come to believe in his innocence.
Maybe he was framed for a crime he didn't commit. Some jealous rival could have set him up to get him out of the way.
One possible reason could be a misunderstanding. Maybe some dark magic was involved that made Harry believe the Weasleys were acting against him or the wizarding world in a way that seemed illegal. For example, if there was a spell that distorted his perception of their actions.
Perhaps Vernon is caught up in a wizarding - related incident without realizing it. He might accidentally help some dark wizards thinking they are just normal business partners. When the wizarding world realizes his unwitting involvement, they arrest him to question him about the dark wizards' plans. It could also be that his mistreatment of Harry finally catches up with him in a legal way. For example, if there are laws in the wizarding world about the proper treatment of magical children in non - magical households.
Perhaps in the fanfiction, Rachel is wrongly accused of stealing a valuable prop from the theater. She was just borrowing it for inspiration but there's a misunderstanding and she ends up getting arrested. Then she has to fight to prove her innocence.