Changes back to default theme itself

I’ve got a wordpress install running here:

http://www.leopardtricks.com/

Every now and then the theme I have on it (iTheme) will change back to the default theme, itself! I’m not sure when it’s happening but I think it has something to do with me being in the admin panel, either making posts, or editing them, or approving comments, I’m not sure which. But it’s really frustrating, I keep having to change it back manually.