Perhaps Snape makes a derogatory comment about Hermione's intelligence or her Muggle - born status in the fanfiction. Hermione is very proud of her academic achievements and her heritage. Any slights in these areas would definitely make her lose her temper and yell at him.
It could be that Snape is interfering with Hermione's attempts to help a friend or protect someone in the story. If he stands in the way of her doing what she believes is right, her passionate nature would lead her to yell at him in anger.
One possible reason could be Snape's unfair treatment of Gryffindor students in the fanfiction. Hermione, being a strong advocate for justice and fairness, might not be able to tolerate his constant bias against her house. For example, if Snape gives Gryffindor students harsher punishments than students from other houses for the same infractions, Hermione would likely be furious and yell at him.
Perhaps Snape was being overly harsh with one of Hermione's friends. She is very protective of Harry and Ron. If Snape said something mean or did something to harm them, Hermione would surely stand up and yell at him. For example, if he gave Ron a particularly difficult detention for no real fault of his own.
Maybe James Sirius Potter did something really reckless. For example, he might have put himself in danger while exploring some forbidden areas in Hogwarts. Harry, having gone through a lot of dangerous situations himself, would be worried and yell out of fear and frustration.
Maybe she's yelling at Ron. Their relationship in fanfictions can be quite complex. If Ron says something insensitive or does something stupid, like forgetting an important task that could put their mission in jeopardy, Hermione would surely yell at him.
One possible reason could be jealousy. Maybe Ginny is jealous of Hermione's close friendship with Harry or her academic achievements. In the wizarding world, relationships can be complex, and Ginny might feel overshadowed by Hermione at times.
One possible reason could be Snape's unfair treatment of Gryffindor students. Hermione, being a strong advocate for justice, might not tolerate his constant bias against her house. She could yell at him when he unjustly deducts points from Gryffindor for minor reasons.
One possible reason could be that Mrs Black's old - fashioned pure - blood views conflict with Hermione's more inclusive and progressive ideas. Hermione, being a Muggle - born witch, might not tolerate Mrs Black's snobbish and discriminatory remarks about non - pure - blood wizards and witches.
Mrs Black could be so offended that she starts to plot against Hermione. For example, she might try to get Hermione into trouble with the Hogwarts authorities by spreading false rumors about her.
Snape might initially be taken aback. He's used to students being intimidated by him, so Hermione's outburst could shock him into momentary silence. Then he might retaliate with his usual sarcasm, trying to put her in her place.
Maybe Adrien yells at Chloé because she's being mean to Marinette again. Chloé has a history of bullying Marinette, and Adrien, being Marinette's friend, might not tolerate it any longer and thus yells at Chloé in the fanfiction.
One reason could be Snape's sense of justice. Despite his often - cold exterior, he might not tolerate Draco's bullying behavior towards Hermione.