Suppose S&P knew it had some sub-prime skeletons which it wanted to protect from any investigation. The Downgrade did that since now any investigation could be characterized as Obama's revenge.

What sort of skeletons?

Consider the initial period of minimal payments in sub-prime loans..

Suppose that wasn't really designed as a "come-on"  to entrap naive buyers. Suppose instead its purpose was to facilitate, say S&P,  providing a triple A to the derivatives based on this junk. Pretty hard for Stuffy & Pompous to do that if they started defaulting immediately.

But no early payments =d no early defaults. which =d a window during which profitably to flog  the derivatives to some naive investors .

Wouldn't S&P have understood that game?  Not if the unholy alliance of realtors and banks made it worthwhile not to understand.

But suppose there are some emails explaining this collusion ... That would make it very useful if Justice were prevented from investigating because it would seem like Obama' s revenge for the Downgrade.

But I guess I've just become a conspiracy theorist in my old age.