One possible reason could be a shift in their loyalties. Maybe they discover some dark secrets about Harry or the side he represents that make them uncomfortable. For example, if they find out that Harry has some hidden motives that go against their own beliefs or values.
It could make Harry feel abandoned at first. He might have relied on them in some way, either for information or for support, and their sudden departure would leave a void. For example, if Severus was providing some crucial knowledge about magic or the dark arts, Harry would now have to find another source.
Perhaps in the fanfiction, Draco and Severus leave Harry because they are forced to by some external dark force. This could lead to Harry having to face new challenges alone, while Draco and Severus struggle against this force in their own way. They could end up being spies or trying to find a way to defeat the dark power from afar.
One possible reason could be a misunderstanding. Maybe they misinterpreted something Hermione said or did, and in a fit of anger or confusion, they decided to leave. For example, if Hermione was working on a project alone without consulting them, they might have felt left out and left in a huff.
One possible reason could be to escape the danger and chaos that might be brewing in England within the wizarding world. Maybe there are dark forces targeting them specifically, and leaving is their only option for safety.
The impact would be huge. Harry has always had this complex relationship with Draco and Severus. With them leaving, he might feel a sense of loss but also a new determination. For Draco and Severus, leaving Harry could be the start of a new chapter in their lives. They could end up in different parts of the wizarding world, encountering new people and situations. This could bring about a change in their personalities, and when they eventually cross paths with Harry again, they might be completely different characters.
In fanfiction, one reason could be his desire for a fresh start away from the memories at Hogwarts. Maybe he feels he has done all he can there and wants to explore new magical knowledge elsewhere.
In some fanfictions, Draco and Severus might be portrayed as hurting Harry for various reasons. It could be due to the long - standing rivalry between Draco and Harry, with Draco's family's pure - blood ideology fueling his actions. Severus, on the other hand, has a complex relationship with Harry. His actions that seem to 'hurt' Harry could be misinterpreted. For example, his strictness in Potions class might be seen as a form of harm, but it could also be his way of trying to protect Harry in a convoluted way.
I'm not a big fan of this sort of fanfiction concept as it goes against the original relationships in the Harry Potter series. However, in the vast world of fanfiction, there might be such works out there. You can try searching on popular fanfiction websites like Archive of Our Own (AO3) or Fanfiction.net.
They might hurt Harry physically in some fanfictions. For example, in a fight scene where Draco uses dark magic he's not supposed to and Severus doesn't stop him in time. Another way could be emotionally. Draco could spread false rumors about Harry, which Severus, as a teacher, fails to address properly, leading to Harry's isolation and emotional distress.
One possible reason could be that she feels she has outgrown the guild. Maybe she wants to explore new magic on her own and believes the Fairy Tail environment has become too comfortable and is hindering her growth.