When you modify a title in your project and it affects other titles, then those titles have been set to use a shared design. Certain workflows, such as copying and pasting titles in your NLE project, can cause titles to automatically be set to share a design. If you frequently need to copy titles between NLE projects or timelines we recommend disabling title sharing. Please see this page for more information:
Enable or Disable Title Sharing
For more information on how to use Title Sharing please see this page: