Maybe it happened because he made a big mistake in a magical experiment or broke a strict school rule. That might have triggered a howler being sent to him. The possibilities are endless in fanfiction!
It could be that Dumbledore got a howler because he was caught snooping where he shouldn't have been. Or perhaps he forgot an important event and someone was really angry about it. Fanfiction writers can come up with all sorts of creative reasons for this to happen.
It could be Severus Snape. He has a complex relationship with Dumbledore. Maybe Snape discovers some of Dumbledore's plans that he doesn't agree with regarding Harry Potter. He then sends a Howler in a fit of anger to express his disapproval.
Well, sometimes it's because he did something really naughty or broke a major rule at Hogwarts.
In many fanfictions, Harry might send a howler when he's really angry or wants to make a bold statement. It's usually a dramatic moment.
Well, it could be that someone was really angry with him and sent it to express that rage.
Perhaps Harry was feeling betrayed or misunderstood and wanted to express his strong emotions directly to Dumbledore through a howler. This could be a way to make his point and demand a response.
In the fanfiction, the Howler could be a warning from an unknown dark force. Dumbledore has been making moves to counter the rising dark magic in the wizarding world. This dark force, not wanting to be thwarted, sends a Howler as a form of intimidation. The Howler could contain veiled threats and references to past events that only Dumbledore would understand.
Maybe Snape did something that went against Lily's expectations or values, and she uses the howler to express her disappointment. For instance, he might have joined a group she doesn't approve of.
Sometimes, Dumbledore might try to control Harry by manipulating the information he gives him or influencing his decisions without him realizing it.
Perhaps the Marauders wanted to warn Harry about something important and thought a howler was the best way to get his attention. It could also be a part of a complex magical game or challenge among them that accidentally involved Harry.
Well, it could be that she broke some important rule or did something that really angered another character, and they decided to send a howler to give her a piece of their mind. Or perhaps it was a misunderstanding that led to the howler being sent to her.