If this is the old versions of Sins then this is probably an issue with newer laptops that have two GPUs, one high end the other low end for minimizing drain on the battery.
IIRC early versions of Sins Rebellion used to problems on such systems.
I had problem on my MSi (GTX 1060 + Intel HD 530) where it didn't detect the available resolutions, hence would run in a low res mode.
Switching from full-screen to windowed mode and maximizing seemed an easy work-around in my case.
Another option worth trying is to enable NVidia's "Run with graphics processor" menu from the old "NVidia Control Panel" application. You could then right click on the desktop shortcut for Sins and select "Integrated graphics" instead of using the "High performance NVIDIA processor". Probably easier (and safer) than to keep disabling your GTX 1080.
However, if it is the current version of Sins Rebellion though that should work fine - probably worth raising a support ticket if Rebellion isn't working correctly.