This is most likely happening due to Office running an updater in the background, follow these instructions to make sure the "popup" wil no longer appear on the screen when this happens.
First load the task scheduler by searching for "task scheduler" Then open the window until you see one similar to this.
After that simply follow these instructions.
Modify both of them to the following in the "Actions" tab:
Add arguments: /b "C:\Program Files (x86)\Microsoft Office\root\Office16\officebackgroundtaskhandler.exe"