So looks like that there is a conflict between AutoCAD 2015 and iCloud sharing feature on Mac. You can treat this as a bug or defect.
The workaround is to put DWG files (and work with them in AutoCAD) outside Documents folder on your Mac and wait until the issue will be resolved by Apple and Autodesk.
If you need to have a copy of your DWG files in iCloud you can copy them after editing to some folder connected to iCloud.

Maxim