I think it might have more to do with health and comfort at this point in time. I seem to recall a couple of interviews with him that mention since his accident he's found it hard to sit for a prolonged period of time, and I figure that's why he ended up getting up and pacing during his reading. It added to the proceedings, true, but still. And, as you hinted to, he is rich beyond all measure and a recognized name so it isn't like he has to go out doing signings anymore.