I've seen this phenomenon in an older video on this site (different baby), and another time on a friend's video of their baby (just "tickling" leaves on a plant, of all things); the same type of laughter where the baby anticipates the event with a suppressed laugh, and then explodes when the events happens.
I wonder if there is something deeper here, or if it is just simple conditioning...or just the way babies laugh at most things.
My sister has a videotape of my niece laughing hysterically every time her dad held up a doll and said "Boo!" What made it particularly hilarious was that she would be totally calmed down between laughs. And every time she still found it hilarious. Babies are so stupid sometimes.
We just took video about an hour ago of my daughter laughing at me pretending to sneeze.
It doesn't matter how stupid it is. If there's something easy to make your baby laugh -- especially when he or she is crying -- you will do it for hours.