Good evening, @razzzy.
QuickBooks can ask you to enter your username and password again if you have explicitly logged off from the Company File or when the login period set has expired. Since you have to close the company file to update the software, the system will automatically log you out to complete the update too. This can also happen if the login fails, multiple users are using the same company file on the same computer, or if the user is using a Windows account. You can check out this article for further information: Stay logged in to QuickBooks company file.
Please don't hesitate to touch base with me here if you need any additional assistance. I look forward to hearing from you again.