People often defend tone after losing confidence in the point.

When the argument is lost, the conversation frequently shifts to how it was said. Sometimes the tone genuinely was the problem. Often it is the last available position, and the person is now defending a procedural point because the substantive one has gone.

Watch for that move in yourself. If you have switched to complaining about delivery, ask whether you have quietly conceded the point. Saying that you were wrong is faster and much less corrosive than winning on manner.