WP_Query doesn't sort alphabetically – why?

Hi guys,

with WP_Query i want to fill an dropdown-field (optionbox) with posts from different categories – that’s fine so far!
But i want the optionbox to be filled alphabetically. But it doesn’t work: Posts fill the optionbox by random (or by release date).

What’s wrong here?
$my_query = new WP_Query( array('category__in' => array(8608,13), 'orderby' => 'title', 'order' => 'DESC' ));

Thanks a lot!