Its just a way of expressing how you found somthing funny, somtimes laugh slightly in disbeleif of seeing somthing happen, maybe somthing that gave you a shock, or observing somthing you dont understand. Ive a bit of habbit of laughing when i see some one kick off, or if ive got in trouble in some way, usually when i see someone going over board i just laugh at that aswell.
What about those laughs you get when your tripping balls and your laughing so deep it starts hurting your chest and the more your try to stop it and the more you question your laughing the harder you laugh?