Dredg falls under the definition of progressive music/rock from what I understand. It's a term that's been used to describe them by numerous music reviewers and critics since before the Leitmotif album. It's even a term that was used by them to describe their music a while back on their old old webpage, which has since changed and no longer has information about their style. So you can take my word on that if you like, or not.
http://en.wikipedia.org/wiki/Progressive_music
One way the term has been applied is to subgenres that have evolved from their root genre by innovating, either through incorporating instruments from other genres or using new techniques within the framework provided by the instrumentation of the root genre to make a new or crossover style. Another is in reference to a gradual build-up of energy within progressive music track or throughout an album.
They do both of those.
In rock, the word usually describes music that expands traditional musical structures by adopting influences of jazz, symphonic, folk and world music. Progressive rock artists often string together the songs so that the entire album will become an uninterrupted musical "journey".
And those.
Plus, there's - http://en.wikipedia.org/wiki/Dredg which describes them as progressive.
Finally, there's - http://www.purevolume.com/dredg - a site who's link can be found on Dredg's main webpage, on the "listen" section to be exact, which describes them as progressive and experimental. If I'm not mistaken the above site was created by them, well, their section I mean, so they even describe themselves as progressive.