Never out of a hat for any of these shows, that would be a deadly idea, particularly with DOI where dangerous lifts are involved. Probably down to looks, height, age, personality, partly on who they got the previous year (moreso on SCD than DOI), teaching style, choreo ability (can't have a weak choreographer with a disabled contestant for example) and also an element of what pairings will attract press attention, possibly part of the reason Vanessa always gets good ones and Brianne back in the original run often got young, hot and single guys as well.