Well, sometimes it's because he did something really naughty or broke a major rule at Hogwarts.
Often, it could be that his actions have upset a teacher or a family member, leading to a Howler being sent. Maybe he failed an important exam or got into a big fight with a fellow student.
The reasons for Harry Potter getting a Howler in fanfiction can vary. It might be that he disobeyed direct orders from Dumbledore, or perhaps he accidentally revealed a big secret that was supposed to be kept hidden. Another possibility is that he was caught sneaking around where he wasn't supposed to be.
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.
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.
It might happen if James doesn't follow Harry's instructions or does something dangerous. Like when he experiments with forbidden magic or disregards Harry's warnings about certain magical creatures.
It could create a lot of drama and tension. Maybe it leads to a big argument or a revelation of some hidden secret.
It could be that someone did something really wrong and Harry needed to express his strong feelings in a very public and intense way. Howlers are notorious for being attention-grabbing and forceful.
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!
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Snape could also be a possibility. If Harry did something that Snape considered a serious offense in Potions class, like using an ingredient in the wrong way that could have caused an explosion, Snape might send a howler to scold him. However, it would probably also be filled with his usual sarcasm and dislike for Harry.
Maybe it's because Harry did something really naughty that Sirius needed to scold him about.
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.