It is funny that you should mention this (pardon the pun) as I was discussing my laughing during times when you shouldn't, to a friend.

My experience is not related to BDSM, but fairly close enough I suppose.
My orgasms are intense and I always end up in uncontrollable fits of giggles.
I am always embarrased by it and have since stopped my orgasm short before the giggling starts.

I looked into why I do this and found out that it is just a natural occurence, there is nothing to worry about it and to let it happen.

During high emotional states, such as an intense orgasm, or intense flogging, the brain and the nervous system become highly excited thus releasing a large amount of dopamine into the brain and converting it into adrenaline. The excessive amount of adrenaline creates a Fight or Flight response to the brain thus creating a strong emotional release, whether it be crying, or laughing.