The word ‘popup’ is a little out-of-date. We no longer use popups for authentication, so that value is in fact used as the <title> of the tab during authentication:
(i.e. this bit:)
crowd_title is used as the label on the login button (if you have multiple login methods enabled)
If you don’t supply crowd_popup_title, then crowd_title will be used for the <title> of the login tab.