When are creating viewports or copying layout tabs and when you double click the viewport to activate it will  zoom extent.
Because you need to change the scale, regen or zoom to a different location your viewport does an automatic zoom extend. If you have a lot of information on your drawing this could become a really annoying problem.
Change the UCSFOLLOW system variable to 0 inside the viewport.

The UCSFOLLOW setting is saved separately for each viewport, paper space and model space, when you create a new viewport it always maintains the setting use on model space.

viewport zoom extent