Basically auto-login feature is enabled for cPanel PhpMyadmin which means the users can login to phpMyadmin from cPanel without any additional authentication.
But sometimes PhpMyadmin may ask for additional authentication.
This may due to the cPanel and MySQL password for that account are out of sync. Inorder to resolve the issue reset the cPanel password through WHM so that everything will be synced.
"WHM >> Password Modification"