Rocking in Bob's chair

Dylan has rocked before, in the Laz-E-Mom chair in his bedroom, but nonetheless looked initially confused when I called this rocking. He looked over to his right and said "rock," pointing at a rock Bob has on his end table. Not sure what the "ya ya" is supposed to mean. He's clearly pointing at the arm, so was that a very bad imitation of "rocking?" He does echo "chair" soon after.