nothing to knock the rocks off either
Well the comet is spinning, so there's that. Any movement would fling objects off of it because there's nothing to keep them there. Also, the question arises as to how they got there in the first place without gravitational attraction. Maybe electrostatic? Comets go way far away and then return to the inner system and go around the sun. If there is a charge differential of some kind between the two poles of the orbit (i.e. the sun vs. deep space) then maybe that's it. I wonder what kind of EM field readings the spacecraft is getting from it. Weird object. NASA should look for a hatch...