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.
There's also a possibility that an old enemy of Harry's, who knows about his close relationship with the Weasleys, decides to use this connection to harm both of them. This enemy could be someone from Voldemort's old regime who is still lurking in the shadows and sees this as an opportunity to create chaos among the good guys by getting Weasley arrested through Harry.
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.
It could be Voldemort or his followers. They might see Dumbledore as a major obstacle in their path to power. So they could manipulate Harry, perhaps through some mind - control magic or by spreading false rumors, to get Dumbledore fired. By removing Dumbledore, they hope to gain more control over Hogwarts and the wizarding world in general.
Voldemort could still be the main villain. He might target Harry as usual, but this time the Weasley twins get involved more directly. They could use their cleverness and inventions to try and thwart Voldemort's plans, like creating anti - Dark Arts devices that can detect Death Eaters.
In some fanfics, there could be a magical creature or spirit that Mundungus has befriended in the past. This creature or spirit, being loyal to him, might try to help him in some way. It could be a house - elf that he once helped, and now the house - elf wants to return the favor. Or it could be a magical bird that can carry messages and tries to find someone to assist Mundungus in his predicament.
It could be Harry Potter himself. He has been a victim of her tyranny and might finally find a way to expose her once and for all.
The Ministry of Magic could be responsible. They might be jealous of Dumbledore's power and popularity. They could see him as a threat to their own authority and come up with some false charges to arrest him. For example, they could claim he was hiding information about a dangerous dark wizard.
Maybe a mysterious new character in the fanfiction. This character could be from Adrien's family's past, someone with a vendetta against his family, and decides to start by getting Adrien arrested to disrupt the family. For example, if Adrien's family wronged this character's family long ago, they see Adrien as a target to take revenge on the whole family.
Obviously, Harry himself would be involved. And of course, there would likely be healers at St. Mungo's Hospital for Magical Maladies and Injuries. Hermione might also be there to support Harry, using her vast knowledge to help understand any complex medical conditions or procedures.
Maybe it's about Harry always looking up to the Weasleys. He might be fascinated by their large family, the way they support each other. He could be constantly seeking their advice or just enjoying their company. For instance, he might love spending holidays at the Burrow with them.
Dumbledore would be a key figure in the reactions. If he believed in Snape's innocence, he would try to find a way to free Snape while also not alienating Harry. He might start his own investigation to uncover the truth. The other Hogwarts teachers would be in shock. Some might believe Harry is right, especially those who have always been a bit suspicious of Snape, while others would be outraged and demand an explanation from Harry.