Spontaneous laughter is usually an involuntary response; it could be completely out of shock. I remember laughing hysterically for five minutes–and then bursting into tears–after hearing that my brother in the hospital would be okay.
Also, shock can drive the air from you, so instead of wailing to start with you can get a sound like ‘huh huh huh’ with gasps in between… could easily sound like laughter while being nothing of the sort. Just another theory.