The CiviCRM installer for WP was changed for version 4.7. It now takes the WP credentials from wp-config.php and pre-populates the credentials. The result is a better UX for non-technical users and is in keeping with the WP style of how plugins should work with the DB.
The intent is to let folks change the DB creds, perhaps point to a separate DB. However, if you change the credentials AND click Re-Check requirements, the database credentials get reset.
We need to track down this and correct.