One possible reason could be the loss of a teammate. In the dangerous world of RWBY, if one of her close friends were to be seriously injured or die, it would be devastating for Ruby. Another reason might be the pressure she feels as a leader. She has to make tough decisions and carry the weight of her team's success or failure, which could become overwhelming at times.
One reason could be failure. For example, if she fails in a magic experiment that she has been working hard on, it might lead to her crying as she is often very dedicated to her studies and magic practice.
He could cry due to unrequited love. Finn has had his fair share of romantic interests in the show. In a fanfic, if the girl he loves doesn't return his feelings, it would be a heart - breaking situation for him. Also, in some fanfics, Finn might be confronted with the harsh reality of the world he lives in. Maybe he discovers something truly dark about the Land of Ooo that shatters his innocent view of it, causing him to break down in tears.
One possible reason could be that she's on a secret mission. Maybe Ozpin sent her on a dangerous errand without telling the others to protect them. For example, he might have discovered a new threat related to Salem and needed Ruby to gather intel quietly.
Blake could share her own experiences of dealing with difficult emotions. Since Blake has a complex past, she can empathize with Ruby on a deeper level. She might hold Ruby's hand and tell her that it's okay to feel sad. Jaune could be a bit clumsy in his attempt but his sincerity would shine through. He might crack a few jokes at first to lighten the mood and then offer some words of encouragement.
One possible reason could be unrequited love. In the fanfic, Chansung might be in love with someone who doesn't feel the same way. This could lead to a deep sense of sadness and cause him to cry. For example, he might have confessed his feelings but been rejected, leaving him heartbroken.
One possible plot could be that Ruby is facing an overwhelmingly powerful enemy alone. Despite her best efforts, she is mortally wounded in the battle and dies heroically. Another could be that she sacrifices herself to save her friends from a deadly trap or a powerful Grimm attack.
One possible reason could be a difference in combat strategies. Maybe Ruby has a very specific and aggressive way of approaching battles, and her team is being more cautious or hesitant. For example, if they keep refusing to follow her lead during a fight against a powerful Grimm, it could make her mad.
Maybe she got lost. It's easy for an old lady to lose her way, especially in a busy area near the signal. She might be confused about where to go.
Perhaps she's being bullied. In the wizarding world, there could be other students who pick on her because she's a 'Mudblood'. This could be really distressing for Hermione, and when Snape finds her crying, he might be conflicted between his general dislike for Gryffindors and his sense of justice.
In the fanfic, Harry might cry because he's now faced with the reality of having to rebuild his life. Before the war, his focus was mainly on fighting Voldemort. But after the victory, he has to figure out things like his career, his relationships, and his place in the new wizarding world. The pressure of all this new stuff can make him cry.