What are the chemical process responsible for the warping of wood?
Warping (be it a bow, crook, cup, or twist); as well as loosening of knots, checking, splitting, and collapse; of wood is caused by uneven shrinkage. 

To explain uneven shrinkage chemically, consider wood as 50% cellulose.  Cellulose, which is a polymer consisting of linked glucose monomers, is arranged into fibrils.  Fibrils are part of larger structures which make up the cell walls of wood fiber.  Binding these cell walls together is lignin, which is a phenylpropanol polymer.  Cellulose and lignin (along with hemicellulose) are the major structural compounds in wood.

Cellulose in wood is a tangle of rigid crystalline cellulose and flexible amorphous cellulose.  While the structure of crystalline cellulose makes it water insoluble, amorphous cellulose has sites like its base monomer glucose which are capable of absorbing water. When water is absorbed both the shape and volume of the amorphous cellulose change.  While amorphous cellulose changes its shape, the crystalline cellulose (as well as lignin) retains its shape and volume.

Cellulose may also change shape and volume due to other factors such as temperature fluctuation and biological degradation.  Because (1) wood is not uniform; (2) moisture, temperature, and biological changes are rarely uniform throughout the bulk of a piece of wood; and (3) most wood is exposed to some degree of these changes over time; uneven shrinkage occurs.  Because of uneven shrinkage in wood, it warps.