This is one of those things I'm already tired of hearing about.
I don't think it was staged at all.
And while I'm not a violent guy myself, I know enough of the social gossip (thanks wife) around Will Smith and his wife to guess that things have been rocking in their house hold and that his wife sounds like she is really struggling since being diagnosed with alopecia. You can tell Will laughs at the joke, but his wife doesn't find it funny one bit, so I can only assume that he sees how the joke effects his wife and then decided to stand up for her.
This was likely something that had been emotional for Will and his wife for months now, a lot of tension and emotions around it, throw in a bit of alcohol (just assuming), and you get a move that is uncharacteristic on live TV.
No excuses, but I'm also not going to roast the guy as I don't know what's going on behind closed doors and what they are dealing with. What I can relate to though is if you want me to try and smack you, say something in front of me about my daughter or wife that I find to be demeaning or 'crossing the line'.