Free-lance mages who were made into bad guys just because the Devs needed more badguys, similar to how they made Necromancers evil in Oblivion.
EDIT: It's unlikely that they worship lesser daedra so much as are friendly with them. Further, since the Magic school of Conjuration was only adopted by the mages guild between 3E417 and 3E427 (most likely as a replacement for the dropped school of Thaumaturgy) they may have been the former practitioners of conjuration. This is a guess but think about it; if the mages guild only started teaching it 12 years before Oblivion (at the max) how could there be masters of it if there weren't orders of Conjurers before?