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 complicated plan
and the Chaos Gods have no idea how much they're being manipulated. The grim darkness of the far future will be 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.
At least one Culture Ship presents with its fields in a saucer-and-nacelles configuration
Because they find it funny.