'Having someone's back' entails that they're in a situation...being attacked...or an attack is being threatened. Will isn't attacking Chris Rock, Chris is in no imminent danger...he has his back how?
Will Smith felt bro disrespected his wife...at the very least he felt like his wife felt disrespected...so he, as her husband, defended her, cut and dried. I'm in the crowd that understands...it's never cool to assault anyone for any reason other than imminent, unavoidable danger in the offing...but I can understand why/how he flipped.
Again, it's not "having his back"...his brother is just upset that he got smacked up on live TV. That's also understandable...but like, wtf are you gonna do about it? Stop talking, offer him the hands or sumn, get that ass whipped for your brother. He's acting like an actual bytch, like a bytch for real, a girl. If you're that tight...still... then get busy. STFU and grow TF up.