Stop the Outlook 2016 Reminders window from being "always on top"
If you switch to the Outlook app then Hide (⌘H) Outlook, the Reminders window will be hidden along with the rest of the app.

If you switch back to Outlook again the Reminders will return to being always on top, so it will occlude other apps again if you switch back away from Outlook other than by Hiding it.

(I figured this out from one of the far more numerous complaints about Reminders NOT interrupting work, where KiltedTim recommended not hiding Outlook)