Wikipedia says an MRAP weighs 14 tons. That's 28,000 pounds. I have to wonder if a tie down strap let go, if the hardware that attaches to the cargo deck was strong enough, some sort of human error in tieing them down.
Granted all of the piloting I have done has just been in little Cessnas and gliders, but from what I recall it was common to get a little bit of rotation or spin during a stall. My guess is that one wing stops producing lift before the other wing. It is an eerie feeling. It's the realization that for a few moments you are no longer in control, that you're just along for the ride.
I am curious if that was the first time MRAPs were loaded on a 747 like that.
You're right about one wing losing more lift faster, causing the plane to drop to that side.
That is how you put a stunt capable plane into a spin .
I also think you are right about the tie downs breaking free .
The C 17 (used to be C 5

) is built to carry M 1 Tanks, Not only are their tie down points over built, the floor they are attached to is reinforced as well as the Air craft frame.
A 747 is built to CIVILIAN specs, for carrying civilian type freight .
On top of that some of the air freight contracts went to some ridiculously shady carriers, such as Victor Bout, who made billions using a fleet of aging Soviet transports to ship guns all over Africa .
And US cargoes into Iraq and Afghanistan.
Since he had been running guns to both the Northern Alliance and the Taliban his crews already knew the routes so it made sense to give him the contracts.
He's in prison in the US but his operation is characteristic of the FBO' getting the contracts.
Just because it's a 747 does not mean it's new.
Or anywhere remotely close to new .

On top of that,maintenance is going to be very iffy just due to parts, on top of that, ground crew aren't always up to our standards .
So it is easy to visualize a poorly tended, overstressed old tie down failing and causing a chain reaction .
More likely than crew error, since even if the ground crew were sloppy the Load Master would have been on it since he's riding with it.