DDR's?

One could as easily wonder why so many varieties of the 2015 NE quarter from the Philadelphia Mint have DDR errors; with no DDRs from Denver, and no circulation strikes from San Francisco are DDRs, (I recall seeing an S proof strike being said to be a DDO. In answering why, I can't help but believe that some of the 2015 activity is Mint assisted. Then you have the comparative complexity of the reverse die being much greater on the reverse with regard to the obverse. And that nothing changes on the obverse, even from year to year, while the reverse changes to a completely new design 5 times each year. And finally: it has been noted that the anomaly of doubled dies has its greatest occurrence within a circular zone at the exact center of the affected die. That being said, this area on the reverse of registered DDRs contains some aspect of the complex design element wherein the doubling occurs while the center portion on the obverse of these coins has considerably less design, or less complex, leaving less to possibly double. Aside that, I haven't got a clue. Cheers.

The design itself is often the determining factor on wheather we end up with a doubled die.
The hub that stikes a working die is in relief just like the face of a coin which means the fields are the lowest point and the last areas to impress a working die.The window pains on the reverse of a Nebraska quarter are basically flush with the field area. This means as the hub and working die (which is cone shaped thus first contact is central) come together , any misalignment will leave an impression in the window area , before the impression corrects itself , and stay there .
Obverse or reverse, in the modern single strike/squeeze era , a field level area in the center is key to giving us doubled working dies.

Basically yes for coins minted after 1996 , although the doubling is at the beginning of the impression instead of at the end and will be shown on every coin that working die strikes.
1996 and before , doubled dies are attributed to different causes because of the softening and re-impression of the working dies.

The first year for the single squeeze method was 1997. The 1997 doubled ear Lincoln Cent was the controversial coin that ushered in the new era of doubled dies.
As far as recocnizing MDed coins nothing has really change . They reduce the overall design and usually apear shelf like. The incuse designes are of course the opposite in appearance as these elements will appear larger.

It's a class 8 (VIII).
John1

There is one that CONECA calls a class 8. But it is not mentioned by John which class number.