One reason could be love. Draco might have fallen deeply in love with Harry and his possessiveness is a way of showing his strong feelings. He doesn't want to lose Harry to anyone else.
Well, in such fanfiction, Draco's possessiveness could show through his constantly trying to keep Harry within his sight. He may be overprotective and not want Harry to engage in any dangerous or potentially harmful activities without his presence. He might also be possessive about Harry's time, wanting Harry to spend most of it with him rather than on his own studies or Quidditch practice.
Well, in such fanfictions, Harry's possessiveness might show in him being overly protective of Draco. For example, he may not like Draco interacting with other people too much and always wants to be around Draco to ensure his safety or just to be close to him.
A plot device often used is the threat of separation. Maybe there's a situation where Draco has to go away for some reason, like a mission or a family obligation. Harry's possessiveness kicks in as he doesn't want Draco to leave. This situation can be used to really dig deep into Harry's feelings and show how far his possessiveness can go. He might try to find ways to keep Draco with him, even if it means going against rules or other people's wishes.
One reason could be to add drama and conflict to the story. Fanfiction writers often like to create complex situations to make the plot more interesting. Another reason might be to explore different aspects of the characters' personalities in a non - traditional way.
One possible reason could be that she sees Harry as a symbol of hope in the wizarding world. As a hero figure, Tonks might feel the need to protect and be possessive of him to ensure his safety for the greater good.
Love is a big reason. She loves Percy deeply and is afraid of losing him. In the world of the fanfiction, there could be many threats to their relationship, so she holds on tightly.
One reason could be that she lost so much time with Naruto due to her early death in the original story. So in the fanfiction, she's overcompensating for that lost time by being possessive.
One possible reason could be a newfound understanding between them. Maybe they've both grown and realized that their past enmity was pointless. They might have had some shared experiences that made Harry see Draco in a different light, and thus he was willing to accept his hand as a sign of peace or new friendship.
One possible reason could be his family's status in the wizarding world. His family might be involved in some dark deeds, and Draco could be a target because of that. Another reason could be related to his own actions or attitudes that have made him enemies.
Perhaps Draco has developed some sort of complex feelings towards Harry. He could be secretly attracted to Harry and treating him like a baby is a way to get closer to him without admitting his true feelings. Or it could be that Draco has changed as a person and now views Harry as someone who needs his support and guidance, just like one would for a baby.