My experience from the D800 and before that from a D200 is that at high ISO combined with incandescent light or even worse, stage lighting with down-regulated incandescent lamps that are even more red-yellowish, the extra amplification of the blue channel needed to get a decent white balance makes the blue channel very noisy. I find that this is what limits the ISO setting. On the D200 the limit was 1600 but on the D800 this problem sets in somewhere between 6400 and 12800, so I am not surprised to see it at 25000.
The noise and dark signal from the detector is temperature dependent and could also vary over the image area. This could perhaps explain why it is predominately at the lower edge in the example.
One way to fix it in the image using photoshop or similar is to apply some color noise reduction and then adjust the black point of the blue channel a little bit so that a completely black area is really black. Then you have to compensate again by adjusting the the gain in the blue because the first adjustment makes the picture look very yellowish. With my D200 I had to do this often, but the D800 i normally work at 6400 and then it is not needed.