It is very difficult to stay polite while getting very angry. Politeness is usually reserved for respectful people. If somebody acts in a way that is perceived as intentionally disrespectful (whether that's actually the case or not), there is a severe psychological dissonance to overcome. Also physiologically people will get nervous, voice shaking, facial tension and twitching, heart racing, mind getting foggy when severely agitated which makes trying to act polite even more difficult. It's easier and seemingly more sensible to just skip straight to snapping or ... bottling the rage up to eventually release it against somebody sufficiently harmless - humans are monkeys after all (which isn't even necessarily bad, we should just strive for civilizing the chimp and strengthening the bonobo within us.)