The Mexican girl was acting stupidly being aggressive towards the other woman. But from the video, she had not physically assaulted her.
The black girl could have made her point verbally without resorting to physical violence. The Mexican surely didn't deserve the beating she got.
I would...
A Mexican girl is bullying another passenger, totally unprovoked:
Then a black girl sees this and confronts the Mexican for her bullying. the taller girls starts to meet out her own justice on the Mexican.
Instant Karma? Or did she go too far?