Well, perhaps there was an emergency at Hogwarts. Maybe some dark forces were approaching, and Snape, being a powerful wizard and a Hogwarts professor, felt it was his responsibility to pick Harry up. Harry was always a target for the dark side. Also, it could be that Snape had some new information about Harry's family or his destiny that made him decide to take Harry under his wing, so to speak, by picking him up.
The teachers at Hogwarts would have mixed reactions. McGonagall might be a bit hesitant, not fully trusting Snape's intentions but also knowing that Dumbledore has some level of trust in him. Flitwick might be more open - minded and wait to see what happens. And the other dark - arts - related professors or those with a more complex view of Snape might be intrigued to see how this situation unfolds and how it might impact the balance of power within the school and in the wizarding world as a whole.
One popular interpretation is that it shows a different side of Snape's character. Maybe in these fanfictions, Snape is shown to have a more paternal or protective instinct towards Harry, which is a huge contrast to his often cold and harsh demeanor in the original books. It could be a way for fans to explore the 'what if' scenarios, like what if Snape had been more open about his complex feelings towards Harry earlier on.
A common theme is redemption. Snape has a lot of baggage in the original story, and in fanfictions where he picks up Harry, it can be a way for him to redeem himself in Harry's eyes. It might involve Snape making amends for his past actions towards Harry and his father.
One possible reason could be that Snape suspects Harry of something dangerous or against the school rules. Maybe he thinks Harry is involved in some secret Dark Arts activities and ties him up to prevent him from causing further trouble or escaping while he investigates.
One possible reason could be Snape's long - standing grudge against James Potter, Harry's father. Since Snape had a difficult past with James, he might project that onto Harry. Another reason could be that Snape is trying to protect Harry in a very strange way. He may think that by rejecting Harry, he is keeping him at a distance from the dangerous wizarding world politics and plots that Snape is often involved in.
One possible plotline could be that Harry discovers Snape has been secretly working against him in a way that endangers his friends, and in a fit of rage, he attacks Snape. For example, if Snape was found to be involved in a plot to harm Hermione or Ron.
In some fanfictions, it could be related to Snape's loyalty to Dumbledore. If Snape believes Harry is endangering a secret plan that Dumbledore has set in motion, he might punch Harry to stop him from doing something stupid. For example, if Harry is about to reveal a crucial piece of information to the wrong person.
It could also be due to some external factors. Maybe there are threats from dark forces that he doesn't want to involve Lily in. If he is getting deeper into the dark arts or some dangerous situations, he might choose to give up on her to protect her from the potential harm that could come her way because of his associations.
One possible reason could be his lingering resentment towards James Potter, Harry's father. Snape and James had a long - standing feud during their school days. Since Harry looks a lot like James, Snape might be transferring his old grudges onto Harry.
One possible reason could be that Snape shows a different side to Harry in the fanfiction. Maybe he protects Harry secretly from some hidden threats that others are unaware of, which makes Harry start to trust him.