I don't think it need be actual anger, it might be repressed memories of unresolved conflict, angry words that should have been said but never were, a sense of helplessness on occasions when others had the upper hand and one couldn't express how one was feeling. A wide spectrum of emotions is behind this, I suspect, but it's largely to do with unspoken sentiments that linger within the body and manifest around the mouth area, where they're still waiting to come out.