Space is full of all sorts of debris too small for us to see, but which will be traveling relatively fast as it orbits the sun. Smack into some of this debris and, depending on the relative speeds between Starman and whatever it hits, a small boulder could deliver a devastating hit to the vehicle or mannequin.
Aside from the debris, Starman is now well outside of Earth’s magnetic field, so the Starman and his Roadster are being slowly stripped clean by the solar winds and radiation, so that lovely cherry red exterior on the roadster has long a bit of its shine over the past year.
When he gets close enough to Earth for us to have a good look at him again in 2091, there probably won’t be much left of him besides the frame of the vehicle, but you know what they say, live fast, die hard. For Starman, it'll have been a hell of a ride.