Swearing is broadly considered offensive or potentially offensive. When you're mad, you don't care what's offensive, or you actually mean to give offense and you wanna amplify it. When you're calm, you consider offensive actions and tend to scale them back. Say I wanna call out Johnny. Mad Uberdawg, apathetic toward offense or aiming for it: "Johnny, you -blam!-in' prick. What the hell are you doing?" Calm Uberdawg, seeking measurement and inoffensiveness: "Johnny, I think that's a mistake. What are you doing that for?" There are, of course, a ton of exceptions. That's just the general thought behind the social custom, I suppose.

Well, I would question why someone felt the need to use such vulgar language while calm. Then again, who really gives a shit?

Because people swear when they are mad. Swearing in normal talk is usually just dumb.

Because of the demeanor it's commonly associated with. An [i]angry[/i] one. I swear just as much as the next person, but when it comes down to it, it's just vulgar language.

I never swear! Even when I'm mad!

Swearing doesn't mean anything any more.