I ended up opening a support ticket to solve this one. Thank you Josh@Nimble.
Cause - not exactly sure. Somewhere something got out of sync and locked a bunch of blocks on the volume that can only be cleared by deleting the volume.
Solution - Storage vMotion the volume to another datastore. Unmounted and detached the offending volume following VMware best practices and then deleted it from Nimble. Recreated the volume on my Nimble, created the datastore and then vMotioned the vmdk back.