There are a bunch of reasons that inform that. On TV, they used to have censor bars over people's mouths as well as "bleeping" what they said. So putting your hand over your mouth emphasizes the obscenity - plus it's funnier :) Also, from a distance no one can see your lips so it gives some plausible deniabilty. It can also make the outburst quieter, which helps when you're fake-yelling.
By the responses I got, I was starting to think that I am the only one who noticed it.
I wonder why it hasn't caught on with other racial groups though.
I've honestly never seen anyone do that..
I've seen it done by teenage girls from the South. That's about it.
I've lived in multiple states in the South my entire life and have never seen anyone do this.