The moderator was faking his laugh. The audience laughed at Bob's stupid giggling. That's why the delay.
Yeah, it is pretty unbelievable that the moderator was faking giggles so that Kerry didn't look like a complete idiot. I don't recall him laughing like at anything that President Bush said - not that he needed to because unlike Kerry, President Bush actually has a sense of humor.
Yes, Bob Schieffer did resemble Beavis laughing at Butthead.
I believe the audience was laughing nervously at the spectacle of a CBS newsman morphing into a jackass.