In the leaf pic, you can see a round that looks like the center is rotted out. I haven't cut much Black Walnut, just a few I've found out there that had been lying around for I don't know how long. Even though they had probably died and fallen over, I don't recall seeing any rot in any pieces. I know that Walnut is rot-resistant, maybe the trunk/branches don't usually rot and hollow out? Someone who has cut more Walnut would know better than I would, though.Heartwood isn't as dark as it should be for black walnut in some pieces, but I wasn't sure if that was the result of rot/decay.
I guess maybe a lightning strike could get the trunk hot enough to evaporate all the moisture. I once saw a bolt light up a utility pole about 100 yards from me. The strike glowed the pole, three times the width of the pole, for about a second...