As his subsequent semi-coherent and over-emotional babble of a speech indicates, Mr Smith is at the end of his tether. We might well see more odd behaviour in the coming months and weeks. The act of slapping itself was decidedly off-kilter, men tend to punch when attacking other men, not slap.
In the opposing corner, we have Mr Rock, a decidedly nasty piece of work if I'm any judge (takes one to know one), who has been jibing at the Smiths, it seems, for some time.
The whole scenario reeks of 'wrong place, wrong time' for both men, and one can only wonder if a third party had a hand in setting things up. Possible, but I'm more likely to go with the idea that Mr Smith suffered a minor breakdown (or the start of a major one).
I'm only too well aware of the effects of mental illness. Well enough aware of my own to know that in Mr Smiths' position, I would have done the same, only a lot harder. That's why I avoid socialising. It's also why I won't be in the same room with my daughter's ex.