Warp is often initiated by differences in moisture from one side of a board to the other. For example, if you lay a board on moist ground, out in full sun, for several days, you will get some interesting warping. The sun creates one very dry side, the ground one very moist side. This same effect may happen when you attach clapboard to the side of a house with no vapor barrier. The sun bakes one side, moist air from the inside of the house works on the other.
Warped siding is the result.
Cracking Logs, columns, and larger dimensional lumber usually crack radially as they dry. Very thick pieces of lumber take years to come to MC equilibrium with their environment. Because radial shrinkage cannot keep up with tangential shrinkage, a crack occurs where excessive tangential forces pull apart wood fibers. Cracks form from the center to the outside. Smaller pieces of stock and kiln-dried wood do not usually have this problem.