'horse laughing' is an hilarious concept. I had noticed this in the international students at my university, how the japanese would always cover their mouths. Curious habit.
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.