Quaoar is an oddball in the belt because of its rocklike density. In one formation scenario, Quaoar could have hit a bigger Pluto-sized body at high speeds and ricocheted off instead of sticking. This may have stripped away Quaoar's outer, fluffy layers.
Found in "On the Fringe" online at: http://www.sciencenews.org/view/feature/id/52225/title/On_the_fringe
Video Credit: Craig Agnor, E. Asphaug