my doc, and physiotherapist says that its a common problem with overweight people, all to do with posture. I have it and its getting worse as I get older...I've been tested for Cushings but it was negative - my endo queried Cushings the first time he met me!
If you need to read up on that, have a look here and maybe you should ask to be tested too.
http://www.cushings-help.com/buffalo_hump.htm