They were laughing because this is an awards show, but a lot of the gag men were in the front cheering SNSD on ridiculously, so they couldn't stop laughing :)
In the 2011 KBS Entertainment Awards, who performed first? Sistar or SNSD?? 'Cause the audience is so quiet while Sistar was performing. I wonder if all their energy faded already because they exerted it all in SNSD's performance.lol