I would zone out and have my eyes glaze over a lot. Few friends that do it. It takes extra to get attention and snap them out of it. Some friends I have to let sit there for a few minutes before even trying to snap them out of it. I would do it when I was getting yelled at.

A lot of people might do this for a variety of reasons, like Amandalouise said. A lot of good information in that post. I'd try working with it rather than stressing out too bad.... but I'm not a professional.