Maybe the pilot was "pedalling" the rudders a bit, something we all have a tendency to do from time to time! Question is though if you pedal the rudders does this set up an oscillation which the yaw damper takes a while to damp out?
By the way, when I talk about pedalling the rudders I dont mean putting huge bootfuls in and then rapidly reversing same but small inputs. Maybe good pilots (like test pilots hehe!) are better at detecting and correcting small amounts of yaw?