Having Hauled a lot of the MRAPS in and out of Afganistan, they are pretty healthy machines. Load breaking loose is the most likely. They are usually pretty well secured, but the military uses a different system than the civilian counterparts. Possible is failure of some components, whether part of the floor system or the tie downs.
The aircraft was climbing way too steeply, a phase where I expect all effort was being made to reduce the AOA. The plane finally rolls off, it may be that this was an intentional recovery attempt, a little more altitude and they would have made it. That makes three 400 freighters lost with their crews in the last several years, all possibly due to load issues, whether fire or shift.
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