got its name because of the stinging feeling one gets if somebody where to put salt on an open wound. the effects of "throwing salt" at a person are usually the same, but the stinging is on an emotional level.
Guy 2: "Man, why you always throwing salt?"
Rick to Chris: Do you know, that Isaiah will move to amsterdam?
Chris to Rick: Oh man you are throwing salt all over the place. Rob shouldn't hear that.