Board Thread:Episode Discussion/@comment-24573226-20171012080158/@comment-4010415-20171020145012

I really think what caused her to cut off Tyrian's tail was her desire to protect the people she cares about and make sure they're okay, even if she herself isn't okay. That's why she kept jumping into the fight despite most likely having no Aura left and said "I won't stand by and watch someone get hurt!"

Bottling up your emotions means that you sometimes don't have a level head, and those emotions can flare up. When Tyrian cut Qrow, Ruby had a temporary flare of emotion that caused her to lash out without really thinking about it. And that's dangerous.