The emotion (anger) has to come out and you don't know how to do it "safely" but you do know how to laugh so that's how you relieve the pressure. I use to burst into tears when I was angry because otherwise I felt like I would "kill" someone and knew that wasn't a good idea? If you are feeling helpless because you don't know how to respond but know you need a response and are aware things are really messed up, laughter might happen. I would find a therapist and work on your anger issues and how to express anger and how to interact with others who are angry with you, etc.