That is in my opinion a glitch in Outlook and Outlook Express and exists as long as I remember using any of these programs.
When either of them checks for the pop account and the login does not work, mostly a connection problem or server sided issue, also login in too short of an interval, the login may be unseccessful and both programs "think" that either the login or the password is incorrect.
Automatically, the tick box to have the password remembered is un-ticked.