Makes me hungry for sunny side eggs! Had the sun do that exactly in Haiti. If I can find the shot I will post it here as well. I kinda like that effect to be honest.
It's the sun overloading your CCD. Some ccd's give the opposite effect and give a dark or black pancake, others are black with small white centre, and some like yours, white flat centre.
I can't explain why but from my observations, CCD blooming is more pronounced if the bright light source falls at the near corners of the frame (sensor).