I don't think there's a full explanation of this one yet, though I think that a lot of the impetus for its use is that
Cadence has been adopted, consciously or unconciously, as the "feminine form" of
Caden.
There are isolated examples of girls named
Cadence in both Australia and the USA in the 1920s. The heroine of Armistead Maupin's 1992 novel
Maybe the Moon was named
Cadence (though much of the time she is called Cady in the book.) There was a character in the 2001 film
Shallow Hal named
Cadence, and the name first entered the SSA top 1000 list in the USA in 2002. A character in the 2003 film
American Wedding probably didn't hurt the growth of the name. It would be interesting to find some of the parents who named daughters
Cadence in 2001 and 2002 and see if they were inspired by the
Shallow Hal character or heard the name somewhere else.