Combination of punky wood and grain run off. Like Pearly first pointed out, there is some significant degree of run-out. A striaght stave cut out of a twisted tree. Now this usually isn't a problem with yew. The oldtimers past here in Oregon used to say that complete runoff from one side of the limb to another is ok as long as it occurs over a span of 12" or more. I have always figured that was referring to an average weight yew bow in a flatbow design.
Some of the wood within the break appears punky to me. Some of the heartwood has that tiny blocky structure within the break (The 7th Picture from the top). That dark line that DC pointed out seems to be the source of some rot.
Like Chris said, yew can be finicky. If someone said they were going to give me some random stave of unknown quality and character that I would have to make a bow to provide food with, and I got to choose what wood species, then I would choose Osage. If someone was going to let me pick from a dozen staves ranging in quality and character, but of all the same species, to make the finest bow I could make, then I would choose yew.