[tobych]

The way I understand it, ignorant of the psychology of swearing but having had to think about it since arriving here from the UK, is that if you're swearing, you're breaking a taboo that people assume you understand, so if you break it, they guess that you've lost your ability to keep your cool, and that because of that they fear you might punch them or shoot them. Which it seems, to me, most people, rightly or wrongly, believe happens quite often here in the US.