Someone thinks you did something and accuses you out in the open. You didn't do this thing, but you can't help but laugh as you try to defend yourself, thereby making you look guilty, which you recognize, but this makes you laugh even more as you continue to half ass your denial of the accusation. Which obviously doesn't help. This process escalates until you're not even defending yourself anymore, you're just laughing at what is now a furious person who is dead certain you did the thing they think you did.