Passwords have not expired and are set not to expire. There are no recent changes to their accounts.
These are all users running Outlook 2016 with Office 365 mailboxes. At its worst, Outlook password prompts are popping up every ten or fifteen minutes.
Starting a month ago, Outlook began prompting a few of my clients for passwords. Feel free to look away if this doesn’t apply to you. This is a frustrating story of a poorly-defined bug with no solution.