Because the appdata share contains files on the cache drive. (Although generally for performance and hydro savings you would generally want appdata to be confined to the cache drive and then back it up)
To fix (if you want it on the array), change the settings to use cache: Yes and then settings - Docker, disable the service. Main - Array Operations, run mover. After its done, change the settings back to "No" and then re-enable docker
To have the share confined to the cache drive, do the above but set it to use Cache: Yes in the first step and disregard the last step