It always starts as a verbal argument or insults are hurled and suddenly Person A makes the first contact. By logic Person B will tend to reply back too.
That very first contact can include either a slap, a punch, a shove, spitting in their face, getting into the person's face and poking your finger at them, etc. I'll give you a brief example:
Person B: Well I'm not stupid nor a c*** like your mother.
Person A: Say what did you just say
Person B: Is backing away but repeats it louder and clear
Person A: Shoves and hits first
Person B: Feels in the right to hit back and does it. Fight continues on and on.
Some people may think Person B deserved that hit but Person B still felt in the right to return it back too and can say ''Well I didn't hit first, Person A did''.
Regardless of how verbally antagonistic they might have been, isn't the person that makes the first physical contact always at fault?