Perhaps Harry discovers some new information about Snape's past that makes him see Snape in a different light. For example, if he learns about Snape's unrequited love for his mother, Lily, and how that love drove Snape to protect Harry, he would feel the need to apologize for his previous animosity towards Snape.
One possible reason could be for misjudging Snape all those years. Harry might have finally realized that Snape was on the good side all along, working as a double agent against Voldemort. This realization could lead to his apology as he understood how much Snape sacrificed and how wrongly he was treated by Harry and others at Hogwarts.
In the fanfiction, it could be that Harry accidentally causes Snape great harm or distress during a magical mishap. After realizing his mistake, Harry would then apologize to Snape. This could be a situation where Harry's spell goes wrong and affects Snape in a negative way, making Harry feel guilty and prompting the apology.
Snape could initially be skeptical. Given their long - standing enmity, he might not believe that Harry's apology is sincere at first. He might respond with a cold glare or a sarcastic comment, like 'Potter, your apologies are as empty as your head.'
One reason could be that James finally realizes the extent of his bullying. In the original story, James often harassed Snape. In fanfiction, he might come to understand how much pain he caused Snape and feel genuine remorse. Another reason could be that he discovers new information about Snape that makes him see Snape in a different light. For example, he might find out about Snape's love for Lily and how his actions towards Snape affected Lily as well.
Maybe Snape was in a really bad mood and Harry happened to be in the wrong place at the wrong time. Snape is often portrayed as a complex character with a lot of pent - up anger, and Harry might have unknowingly triggered it.
In a 'Harry apologizes to Parvati' fanfiction, the reason for the apology could be that Harry misjudged Parvati. He thought she was being nosy when she was just concerned about him. He goes to her in the Hogwarts corridors, pulls her aside, and sincerely apologizes. Parvati, being kind - hearted, accepts his apology and they end up becoming better friends, sharing secrets about their families.
Perhaps Snape discovers some dark plot involving the Dursleys that could harm Harry more than he initially thought. The Dursleys were known to be cruel to Harry, and Snape might have found out that they were involved with some Death Eaters or dark magic users who planned to use Harry in a harmful way. So, he decides to rescue Harry to prevent this from happening.
Maybe they want to show Snape some kindness. Snape is often seen as a strict and unapproachable figure. Harry and Draco might have realized that beneath his tough exterior, he has a soft side. Sending a teddy bear could be a symbol of their new - found understanding and an attempt to break the ice with him.
Maybe Snape was trying out a new, experimental spell and it backfired on Harry. He could have been aiming for something else entirely, like a simple transfiguration for a lesson, but got the spell wrong and instead of changing an object, he changed Harry's gender.
One possible reason could be that in this fan fiction, Harry has developed a deep respect and a father - like bond with Snape over time. Maybe Snape has been secretly mentoring Harry in potions and has shown him kindness and understanding that Harry has never really received from his own father figure. This has led Harry to subconsciously view Snape as a father figure and the moment in potions class is just when this feeling overflows and he blurts out 'dad'.
One possible motive could be that Snape is still deeply influenced by his loyalty to Voldemort in this fanfiction. Maybe Voldemort has some hold over him that forces him to do the unthinkable. Another reason could be a misunderstanding. Snape might believe that Harry has turned to the dark side or is about to, and he decides to take preemptive action to stop what he thinks will be a great evil.
One possible plot could be Percy realizing how much his family means to him after leaving them for his career. He might come back and have a heartfelt apology scene, perhaps during a family gathering like Christmas at the Burrow. He could list out all the things he's done wrong, like being too self - centered and ignoring their love and support.