Are you upside down or am I?

No, this picture did not get flipped around. Our fish likes to float upside down inside our rock tunnel. He knows it is wrong and he doesn't like to get caught doing it. Everytime I see him, I run to get the camera and by the time I am back he has flipped back over and acts all innocent. But I started putting the camera right beside the tank so I could catch him in the act. I am lucky to have caught this one because he flipped back over before I could take another one. I think it is some kind of self-stimulating behavior. What do you think? Why would a fish do that and not like being caught doing it?