Can a punch to the face do damage?

I know someone who spent 6 years in prison.  He was supposed to be sitting in a bar with a friend, having a beer.  Then a fight broke out (nothing to do with this guy or his friend).  This guy suddenly gets punched in the back of the head, he turns around and punches they guy that hit him back.  The guy falls, hitting his head on the bar foot rail, dying pretty much instantly.  They refer to this as a one punch homicide.