I'd say that you have burn-in. The reason is that when you shut-down you probably have a flash and a bright spot in the middle, since the TV closes down too fast. That made the pixels discolor prematurely.
Usually shouldn't happen, so it's always possible the quality of the pixels isn't really good and subject to burn-in on the long run. Not unexpected from those last-generation tubes.
Also, a problem in the Horizontal Deflection at one moment in time could have cut the yoke, making a bright spot in the middle. It doesn't take long for pixels to fade when this happens and the tube is almost new.