As wood drys there are stresses in the wood. Wood is not homogeious the difference between the wood produced while growing fast is different than when the wood is growing slow. Evidence of these different woods show up as the growth rings. Generally the dark portion of the ring is slow growth wood, the lighter fast growth (not all trees are that way). They dry at different rates.
And the wood towards the outside and ends drys first
I'm sure you've seen 1" board that cups as it drys and looks like it could be part of a wooden pipe. That's caused by the different drying rates and the different shrinkage rates of the different woods (some people call the different wood summer and winter wood). The bottom of the cup is always toward the outside of the tree.
As wood drys the ends and outside dry first, which causes cracks in the wood. In essencence the outside becomes smaller than the inside and that situation is not stable. There's hundreds of books written just on that subject and how to deal with it.
On a tree that leans the down side wood is different than the up side. Compression wood verses tension wood. Boards cut from leaning trees will tend to bow as they dry (and as they are sawn).
Limb wood is notorious for these kind of stresses as are burls.
The lower part of a tree (stump end) is different than the upper part of a tree. It has had to grow and stay alive with a whole tree standing on it.
The drying puts stress in all directions. Over 50% of the weight of green construction wood (fir, pine, hemlock) is water. As the water leaves the wood shrinks. (Some trees have more water weight as they do wood weight).
At the very center of a tree is the pith, another kind of wood and the weakest in the tree. All the stresses center there and are pulling different directions. Not right, not left, not up, and not down, but all directions. As a tree, all is in ballance, but when cut and the drying starts with the different shrinkage rates and different woods the ballance is upset.
If you cut wood with pith you release all the stresses and the wood will split. Great for fire wood. But add rotary motion and steel cutting tools and things could fly.
This is long enough but I hope it helps.
