It's impossible to know *why* it happened, but that is a pretty typical stall.
Cause could be mechanical or electrical malfunction, procedural error, cargo shift, or possibly weather (unlikely). We may never know.
One thing to consider. In places like Afghanistan, where every Tom, Dick and Harry has their own surface to air missile, planes usually try to gain altitude as quickly as possible. This could be a contributing factor.
Edit: I have witnessed with my own eyes, seven major aircraft incidents.
