Bowing is a convex warp or curvature of a book's covers, away from the middle of the text block. Bowed covers occur less often than splayed covers. There are many causes of bowing:
Stresses in the boards' directional grain can warp them.
A change in the boards' moisture content can cause their directional grain to warp in one direction.
Leather, vellum, cloth, or paper covering the boards can shrink more or less than the boards and warp them.
Pastedowns can shrink more or less than the boards and warp them.