Is the poleward migration of the maximum intensity of cyclones a result of tropical expansion or are there some other contributing factors?
You will find the full text of Jim Kossin's migration paper here.
http://www.ssec.wisc.edu/~kossin/articles/nature13278.pdf
In it he describes his findings only as an empirical observation.  He does state that the migration is consistent with tropical expansion but he does not attribute the migration to the expansion. In fact he does not get into the subject of attribution at all. Perhaps attrbution will be the subject of a future paper. You may wish to check his list of publications from time to time to see if he has written an attribution paper. His (vey impressive) publication list may be found at http://www.ssec.wisc.edu/~kossin/pubs.html