Yes, you are describing the effect, but not the cause.

I have heard that wood disks can be dried without splitting, if its
done very very slowly over a couple of years. So this to me says
something about the rate of drying being important to understand, not
just the anisotropic behaviour or wood radially and tangentially.

Which is why I was saying it seems that the drying process is faster on
the outer rings of the disk, since its more exposed with additional
surfaces (or other properties of those layers of the wood are more
prone to drying for some reason).

Dean