Well, perhaps Harry discovers some dark secret about Dumbledore that makes him feel he must escape. It could be that Dumbledore is being controlled by some dark magic and Harry realizes he's in danger around him.
Hermione could be a likely helper. She's smart and resourceful, and she would likely believe Harry if he told her his reasons for escaping Dumbledore. She could use her vast knowledge of magic and spells to create distractions or to find ways to bypass Dumbledore's security measures.
Well, it could be that Dumbledore has some sort of hidden device or spell that he activates when kidnapped. For example, he could have a charmed object that sends a signal to his allies, like the Order of the Phoenix. His allies then come to his rescue and he manages to escape during the chaos. Also, Dumbledore is extremely intelligent. He might outwit Grindelwald by engaging him in a complex conversation full of riddles and misdirections, which gives him an opportunity to break free.
Fred and George Weasley are very likely candidates. They are known for their love of pranks and their mischievous nature. They might see pranking Dumbledore as the ultimate challenge.
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.
Snape can be a prominent character. Given his complex relationship with Dumbledore and Harry, his perspective could add a lot to the story. He might have some insights into Dumbledore's actions that Harry doesn't know about, and his reactions to Harry's blaming could vary from being scornful to having a moment of understanding. Luna Lovegood could also be a part of it. Her unique view of the world might make her offer some different viewpoints on the situation between Harry and Dumbledore.
Dumbledore might be motivated by his over - arching plan to defeat Voldemort. He could see Hermione as a sacrifice that needs to be made. Maybe he believes that by betraying her, he can gain some crucial information from the enemy that will ultimately lead to Voldemort's downfall.
Well, obviously Harry Potter. He's the main protagonist in the original series and in the bashing fanfictions, he might be the one to finally see through Dumbledore's facade. Ron Weasley could also be popular. He's Harry's best friend, and in these stories, he might support Harry in his doubts about Dumbledore. And then there's Voldemort. Some fanfictions might use Voldemort as a tool to show how Dumbledore's actions or inactions affected the battle against him. For example, if Dumbledore was so secretive, it could have made it easier for Voldemort to gain power.
In some interpretations, one possible crime could be his manipulation of Harry Potter. He knew Harry was a tool to defeat Voldemort from the start and perhaps didn't fully disclose this to Harry, leading Harry into a dangerous life without his full knowledge.
He might be motivated by a desire to protect Harry's reputation. Fanfiction can be full of wild and untrue stories, and Dumbledore doesn't want Harry to be misrepresented.