Another reason could be related to a competition. If there's a competition at Hogwarts, like a wizarding debate or a magical project competition, and Draco has been using underhanded means to try to win. Percy, who values fairness and order, discovers this and sends a Howler to Draco to condemn his unethical behavior.
Perhaps Draco has been bullying some of Percy's friends or younger students that Percy feels responsible for. In fanfiction, the relationships can be tweaked a bit. Percy might be more hot - headed in some versions and not put up with Draco's pure - blood supremacist attitude towards those he deems as 'lesser' students. So he sends the Howler as a strong warning.
Well, it could be that Draco did something to cross Annabeth in the fanfic's plot. Maybe he messed with one of her plans or said something disrespectful about her friends.
Well, it could be because Hermione did something that Annabeth considered a betrayal in the context of the fanfic. Maybe she broke a pact or made a decision that went against Annabeth's plans.
He might be initially shocked. Draco is used to being the one in control and having others be intimidated by him. A Howler from Percy, someone he may not have considered a huge threat before, would be unexpected. He could be angry and vow revenge in his mind.
It could be about Draco's performance at Hogwarts. Maybe Lucius is disappointed in Draco's grades or his behavior towards other pure - blood families' children.
Perhaps another student, like Hermione, could send the Howler in a very creative twist in the fanfiction. Maybe she has found out that Draco is planning something malicious against the Gryffindors and uses the Howler as a way to threaten him into stopping.
It could lead to all sorts of drama and conflicts. Maybe Draco is embarrassed or gets angry and retaliates.
He'd probably be shocked at first. After all, getting a howler is quite embarrassing.
Well, it could be related to Percy's past actions. If he has a history of being unreliable in the eyes of the character rejecting him, that would be a valid reason. For instance, if he has broken promises in the past. Or perhaps there are external factors at play. Maybe the family of the person rejecting Percy doesn't approve of him, so they influence the decision. Another aspect could be that Percy is too different in terms of values. If he values adventure and risk - taking above all else and the other character values stability and safety, it can lead to a rejection.
It might be because Snape said or did something hurtful to Lily's friends. Lily is a loyal person, and if Snape insulted or harmed her friends in some way, she would be furious enough to send a Howler. In the context of fanfiction, there could be a new character introduced who Snape treats badly, and this angers Lily.
Well, in the fanfiction, Draco might be publicly shamed by the Howler. It could reveal some secrets he's been hiding, like his true feelings about his family's actions in the wizarding world. Maybe it scolds him for not living up to the pure - blood expectations.