Maybe Molly was really angry at Hermione for something she did. That could be the reason for sending the Howlers.
It could be that Hermione crossed some line that Molly felt strongly about. Maybe it was related to Ron or something in the wizarding world that Molly felt Hermione was involved in the wrong way. So, she sent the Howlers to express her displeasure.
Possibly, Hermione's actions or decisions were seen as going against Molly's expectations or values. It might have been something serious enough for Molly to resort to sending Howlers to get her message across forcefully.
Maybe she's trying to scold him for something he did wrong or to give him a stern warning.
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.
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 in some fanfictions, the writer wants to create conflict. Molly might be written to hate Hermione because of differences in their personalities or values. For example, Molly is very family - oriented and Hermione is more focused on academic and magical achievements. Their different priorities could lead to friction in the fictional stories.
Perhaps in the fanfiction world, there are some misunderstandings. Molly could have accidentally said or done something that Hermione misinterpreted. Maybe Molly made a comment about Hermione's Muggle - born status in a way that Hermione took as offensive, even if it wasn't meant that way. This could easily plant the seed of hatred in Hermione's mind.
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.
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!