Unqualified pilots and wake turbulence according to the NTSB.
http://www.dallasnews.com/sharedcontent/dws/news/world/mexico/stories/111508dnintcrashfolo.1b272a695.html
Odd article, this: the headline quotes the NTSB and the text quotes the Mexican Government. Isn’t it also highly unusual to release CVR information? Seems to fall back on the old standby of blaming the pilots, too. Sounds like the NTSB (or whoever) have excelled themselves.