Just guessing, but...
1 - The app always starts in the main window when any of the accounts has an auth error.
This is to aid users with changing account passwords (too many seem to forget they have "start the app in", and unable to get into the main window by pressing the Back key).
2 - By Android convention, "exiting" an app with the Home key and coming back is supposed to preserve the app's state. "Exiting" with Back and coming back brings the UI into the "initial" state... for AquaMail, this will include the "start the app in" setting.