I have an 11 year old who is diagnosed with AS, and a 7 year old who is not AS (but not too far from it.) The 7 year old laughs when his brother gets really upset. It's pretty clear to me that it's a nervous reaction; a coping mechanism. His big brother can be really intense and irrational when he's upset, and I can see that it's hard for the 7 year old to know what to do with those emotions, so it comes out as laughter. Then his brother gets more upset because he thinks he's being laughed at, and it spirals downhill from there.
I think it's pretty common for people on the spectrum to have unusual or inappropriate ways of showing emotion. When I was in basic training, in the military, there were a couple of people who smiled when they were being yelled at, and got yelled at even more. Later on, when we were away from the drill instructor, they would say, "I get so nervous, I can't help laughing. Then I get even more nervous because I can't stop, and that makes me laugh even more." I have no idea if those people were on the autistic spectrum or not.