Harry Potter might send a howler when he's had enough and wants to get his point across loudly and clearly. It could be in response to a betrayal, a false accusation, or to put someone in their place. The way it's depicted can vary greatly depending on the fanfiction author's imagination.
Well, it could be that Harry sends a howler to express his frustration with someone's actions or to correct a misunderstanding in a very public and forceful way. Sometimes it's to stand up for a friend or to defend his own honor.
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.
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.
Well, sometimes it's because he did something really naughty or broke a major rule at Hogwarts.
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.
Maybe Harry was really angry or frustrated with something Sirius did and wanted to express his strong feelings in a dramatic way.
It could be that Molly crossed a line with Harry, and he felt that a howler was the only way to get his point across and make her understand his perspective. Maybe it was a big misunderstanding or a serious rule violation on Molly's part that triggered Harry's reaction.
Maybe Harry was really angry or frustrated with Sirius and wanted to express it strongly. A howler gets the point across!
Maybe Harry was really angry and wanted to shock Voldemort. A howler is a bold way to show that anger!
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.
It could create a lot of drama and tension. Maybe it leads to a big argument or a revelation of some hidden secret.