Why does the security question pop-up keep appearing in Outlook?

The security question will appear whenever a secure email is selected within your Inbox or Sent items lists.

If the top item in your Inbox is a secure email, this will be selected by default whenever you open your Inbox and the pop-up will appear.

You can cancel the pop-up by pressing the Esc key or by navigating around your Inbox using the Up and Down keys.