Why play fight may turn into real fight?

[CatFan]

https://www.youtube.com/watch?v=xhMVQZNujGY&feature=youtu.be

In this video two women in reality show decided to wrestle in the oil.But then the fight got out of control
I think may be there was some rivalvary between them in advance.But which kind of other reasons may appear during the play wrestling in order to transform into real figh