What if he wants to dance with her? I think his attempt later in that same chapter to give up his chair for her is meant to try to draw her out into conversation, but she is obviously mortified by his attempt. We have Anne's judgement of Wentworth's feelings for her at this point not the narrator's. I'm beginning to believe that they are prejudiced by Anne's own self-debasement.

First of all, I'm sure that he notices that Anne is being used for her musical abilities and has to know better than the careless comment that she never dances and never tires of playing. FW for a fact knows that isn't true. Maybe he feels like she's hiding behind the piano to avoid him. I think it's possible that the fact that Anne has withdrawn from him has intrigued him and made him want to try to reach out to her.