I think I understand Mouse, though I can't put words on it properly.
I've had things (not necessarily laughter) come spontaneously from unknown parts of me in therapy. It's an odd feeling.
I've also laughed in therapy when I've known at some deep level that I'm really crying.
The other thing is that laughter releases oxytocin and the like. We can learn to make ourselves laugh in order to feel better. Maybe the laugh came from a part of you that learned that long ago...?