Hi there, we have had many experiences with this tail chasing drama, ( 18 Plus Police pups ) and throughout our fostering we were instructed clearly not to permit this behaviour, apparently at some point in a GSD life area that tail becomes a fine chasing target, and taking into consideration that these here GSD's are fairly focused on seeking and retrieval, from our experience it is an attention or boredom getting moment, we cured this by re-directing the focus with a raggy, or any other controlled toy, by that i mean having a special toy that the pup only gets to play with when you permit, over time i have found that leaving permanent toys around create complacency with the GSD's, but if you have controlled play with a fovourite then your dog and human bond will be very rewarding, another way that we started our no tail policy was to get the pup to sit face on and gently pat the throat and indicate that this behaviour is not accepted (with kind words of course) then directly did some short lead walking
hope this helps some
Dirk