I've seemed to notice it's when someone is being ignorant. I can't go into all the technical biological details as some here on the chemicals involved but I have noticed it's when someone is being ignorant. Name any instance where a laugh can't be contributed to it.
Now as to why, I guess it comes down to man just being more light-hearted to someone who displays it. Since the person isn't perceived as a threat, it makes us feel better about them and more trustworthy since we can see an inherent weakness. And also our parenting side comes into play giving us perhaps a need to take care of the person or befriend them. But don't get me wrong on the ignorance, it's not meant in a derogatory way. A kitten is ignorant of the world when first born just as we all are yet it makes us laugh or smile. Someone does a clumsy action portraying, in a sense, they are ignorant or unobservant of things around them. And if they're not hurt, we may laugh and feel a caring for them since we see a weak side of them.
Yeah, I'm no neurologist, but no one else responded so I thought I would give you my 2 cents.