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 Sirius did something that Harry saw as a betrayal or a serious mistake, so the howler was Harry's way of giving him a piece of his mind. Maybe there was a misunderstanding or a conflict that needed such a bold communication.
Maybe it's because Harry did something really naughty that Sirius needed to scold him about.
Maybe Harry was really angry or frustrated with Sirius and wanted to express it strongly. A howler gets the point across!
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.
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.
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 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.
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.
His father, Harry Potter, might also send a Howler. Harry knows the importance of discipline and responsibility, and if James was shirking his duties or not following the rules at Hogwarts, Harry would not hesitate to use a Howler to communicate his disappointment and expectations.
It could be Professor Snape. He often had a contentious relationship with Harry, and if Harry did something that Snape thought was particularly wrong or against school rules in a way that affected Snape's interests, he might send a Howler.