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.
I think Molly might have gotten a howler because she was involved in a controversial situation and someone wanted to express their strong feelings towards her in a dramatic way. Maybe it was related to a family matter or a magical incident that caused a big stir.
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.
One possible fanfiction idea could be that Molly's howler in Harry Potter was actually a warning not just to Ron, but also a veiled threat to a dark wizard who was secretly infiltrating Hogwarts. Maybe this dark wizard had a plan to use Ron as a pawn, and Molly sensed it through her motherly instincts and magical awareness. So the howler was her way of sending a message to both Ron and the hidden enemy.
In a fanfiction, the howler could have had a physical form beyond just a shrieking letter. It could be that when it arrived at Hogwarts, it took on the form of Molly herself for a brief moment. She could have given Ron a stern look and a lecture in person (sort of) before disappearing again. This would add a whole new level of drama and magic to the concept of the howler in the Harry Potter universe.
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.
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.
There are various types. Some might focus on the emotional impact of the Howler on their relationship. Others could explore the mystery or adventure that ensues from it.
Well, it could be that someone was really angry with him and sent it to express that rage.
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.
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.