I think it could be either one of those. At the greatest deflection (corners of the screen), the magnets in the yoke are at their highest voltages, in order to change the direction of the stream of electrons the greatest amount. If the high voltage isn't available, then the deflection coils can't supply sufficient voltage to deflect the electrons all the way out to the corners.

I think, but am not certain, that this could be caused either by low supply voltage or by defects in the components on the analog board that process the power before it goes to the yoke.