Same things I was "leaning" (get it...head tilt...its leaning...never mind) towards. I've seen rats end up like this because of a fall from inside their enclosure (usually its a multi-level enclosure though) or because of an inner ear problem. I had a mouse that would run in circles all day every day, only stoping to eat, drink, poop, and mate. It could not track a straight line, only circles due to inner ear issues.