My pet theory after reading that: He just has really, really crap form, and his punch was thrown with a slapping motion so his curled fingers impacted rather than his knuckles.

The witnesses, at a loss to figure out how to describe such a stupid move used to do something so horrible, called it a "closed hand sort of slap."

I like this theory because he is a professional boxer apparently, and so him being revealed to have terrible form outside of a controlled environment destroys his image.