Switching applications with CMD+TAB launches an instance of the target app
Could it be that you are pressing Option at the same time?

There's a little known feature that can be used to open new windows in (some) apps from the macOS app switcher.

1.Cmd+Tab to the app of your choice
 2. Keep Cmd held
 3. Press and release Option
 4. Release Tab  

https://lifehacker.com/384167/open-new-windows-via-command-tab