As of the most recent Office 2016 for Macs was released, now all of my users are constantly prompted to reenter their passwords when there is a change in machine state. Meaning, if you close your lid at your desk, go to a conference room, open the lid, you will be prompted to re-enter your password. This occurs if you change SSID's as well. Again, specifically on the Mac platform.
After opening several tickets for my users, Microsoft support states this is known behavior and due to EWS that Mac Office uses to communicate with Office 365. Sadly, I don't believe this as this issue only surfaced after the last update which clearly has some bugs or miscoding that reset the connection timer on the Macs from the previous setting to zero. This then forces the client to re-authenticate.
If the users press Escape to the dialogue box, the warning goes away and Outlook remains connected to Office 365. Very annoying and again, this has only been a recent issue since the most recent office update and hope Microsoft will own the issue and fix for the rest of the Mac users with Office 365 installed on their computers.