Apparently most of the pros use outside choreographers for their dances, at least for assistance.
Sometimes the use of an outside choreographer is due to timing - the pros are dancing, teaching, training constantly so don't always have the time to choreograph, so they use outside or retired pros to help out -the outside person choreographs (often with input from the pro, or the 'theme' given by the pro) and the pro teaches their partner. Some pros are also really good at dancing and teaching, but shite at choreography. I mean, most figure skating coaches don't also do choreography...
Sometimes it's due to choreographic skill, and sometimes it's because some of the pros don't have their background in some of the styles, so use the help to make sure the dance fits the style. For example, Lacey Schwimmer's background was in Swing, and Allison Holker's was contemporary.