Perhaps he broke a promise or didn't support her when she needed it. These kinds of things can really make a girl mad at her boyfriend in a manga.
Maybe he forgot an important date or said something hurtful.
Maybe she's mad because of a bad ending to her favorite story.
A mad anime girl in manga typically has erratic behavior. Maybe she lashes out randomly or has a short temper. Also, her eyes could be drawn in a way that shows her fury. The way she interacts with others is often unpredictable and intense.
A 'mad manga girl' can be appealing because of her unique personality and unpredictable actions. It adds excitement and keeps readers engaged.
There are various types. Maybe a shoujo manga where the girl is head-over-heels for someone and shows it through intense emotions and actions.
The manga girl might be angry because her boyfriend didn't support her when she needed it. Or maybe he was too busy with his own stuff and ignored her. Who knows, but there are lots of possibilities!
Maybe he made a big mistake and hurt her. That could make him regret.
It could be that she met someone who seemed more exciting or offered something her current boyfriend didn't. Maybe she's going through a personal crisis and makes a bad choice.
It could be a fun and unique concept. Maybe the girl has specific requirements or goes through some interesting adventures in finding the 'right' boyfriend.
Maybe it's a fantasy manga where the girl has some magical or unique way to order a boyfriend and faces adventures because of it. Different possibilities exist depending on the author's imagination.
Maybe it's because of her expressions or the situations she's in. Sometimes, the way the artist draws her can give that impression.