It is speaking negatively about someone else. I do not agree that it must be to their face. I do not agree that it must be before a fight. It could be- But It also could be without the intent of the person you talked about ever knowing what you said - or not caring either way. It is just negative talking - and with the things said being true or not in a disrespectful way. I know this as a fact.
"I heard you have been talking smack about me!" (it has been said that you have spoken in a disrespectful way about me.)