I run into shake often, and you will too Jeff. I can't see it in the photos though. Ring shake, or "wind shake" as it's often called by old timers, is generally thought to result from a combination of what jefferey stated, a bacterial infection which weakens the tree, then another applied force which stresses the tree. Such as wind, felling them with heavy machinery, and in some cases steep growth angles.
Shake is the separation of the growth rings, parallel to the rings. I see it here mostly in walnut, white oak, and honeylocust. A tree can have some separation but not have it throughout the tree wholly. It tends to occur more frequently where the sap and heart woods meet, but certainly not limited to that area exclusively. I have some logs in the yard with shake so bad I'm not going to mill them.