Why they call them "cat fights"
Everyone probably refers to your characters fighting as "catfights" because, well, they fight like two girls in a fight. Hair pulling, grabbing, and so on.
Boys usually model their fights after whatever gladiatorial sport is popular in the media at the time.. well, after (and usually before) a lot of posturing and puffing up.
Girls don't usually treat a fight as a valid way to sort out social hierarchies and to gain social status and face. Instead, they fight to beat the hell out of the other person.
@JZ: Just like a cat? your explanation explained nothing.
According to the Wiki: Catfight (also Girl fight) is a term for an altercation between two women, often characterized as involving scratching, slapping, hair-pulling, and shirt-shredding. It can also be used to describe women insulting each other verbally. The catfight has been a staple of American news media and popular culture since the 1940s, and /use of the term is often considered derogatory or belittling./"