Was happening today and I was paranoid that I had a virus or something.
Dug around and found a couple of reddit threads, but TLDR:
Task Scheduler > Microsoft > Office > Right click "OfficeBackgroundTaskHandlerRegistration" > Disable or Delete.
Seems to be various solutions but I've gone with the solution above.
https://www.reddit.com/r/Windows10/comments/6dnsb4/today_i_n…
https://www.reddit.com/r/buildapc/comments/6dqj1b/since_yest…
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_…
Thanks spaghettiman!