Hi,
I have never, ever, heard of a Unity system spontaneously resetting a password. I don't think it's possible. I suspect somebody reset the user's password.
Open the Unity System Administrator, and go to that subscriber's Phone Password Setting's page, and see what the "Date of Last Change" field is set to. Then go to the Reports > System > Administrative Access Activity Report, and run the report for that day. The report is kind of hard to read, but you might be able to figure out who reset the password.
Nancy