Well, it could be Ron. He can be a bit hot - headed sometimes and might be the first to get jealous and start something. But it really depends on how the author of the fanfiction has written it.
'Out of the Cage' was not finished yet. Docs [2],[3],[4], and [5] all mentioned that the novel was still being serialised and mentioned the latest chapter update time. Therefore, it could be concluded that 'Out of the Cage' was not finished yet.
One reason could be love. Hermione is a smart and kind - hearted girl, and the Weasley boys might all develop feelings for her. Jealousy would then be a big factor in their fights.
Here's another. In a college campus, Jane and Mary were attracted to a popular boy named Jack. They were both in the same club as Jack. One time, there was a club event where they were supposed to choose partners for a team activity. Jane and Mary both made a beeline for Jack. They started bickering about who had known Jack longer and who had more in common with him. Jack was caught in the middle and finally just chose another girl to be his partner for the activity to avoid the drama. As a result, Jane and Mary were left embarrassed and realized their fight was rather silly.
Whether MSA are real stories is not straightforward. Sometimes they draw inspiration from real life, but are embellished or fictionalized to make them more engaging or to fit a certain narrative.
Well, it's hard to say for sure. Some MSA stories could be real, reflecting actual happenings or people's true accounts. But there's also a possibility that they're made up to captivate the audience and take them on a fictional journey.