The Culture/WMG

The GFCF of Surface Detail is a SC front
What better a way of having plausible deniability?

The Culture and Warhammer 40,000 exist in the same continuity, and the SC Minds are screwing with the 40K world
The Minds are working on an immensely complex Xanatos Roulette and the Chaos Gods have no idea just how much they're being manipulated. The grim darkness of the far future will end up assimilated into the Culture.
 * There are hints (or, in the newer books, it is nearly outright stated) that the Tyranids are running from someone. They are running from the Culture and their Allies.