Problem Definition
Presently in Bitbucket Server, when creating a fork, the checkbox to Enable fork syncing is checked by default. 

Suggested Solution
When Forking "Enable fork syncing" checkbox should not be checked by default. A global or project/repository level option could be added to toggle the default state of this option. 

Workaround
The only workaround is to manually uncheck the Enable fork syncing if this is not needed by developers.