Just looking at the lack of skid marks, it actually looks like this may be driver error (my estimate). From the photos and video that were made available, it looks like the HMMWV slid along the guardrail and before the driver was able to correct, the passenger side front tire caught one of the vertical uprights and the whole front hub was ripped off at the ball joints (the weakest part) - sending the wheel and hub down the embankment into the ditch):
The direction of the scrapes from the b-pillar, windshield, and driver's side of the vehicle on the road show the main direction of momentum of the vehicle, which tells me (admittedly an arm-chair accident reconstructionist at best) - the truck was travelling on the right shoulder, not in the right most lane of traffic. This supports the guardrail strike theory:
What is unfortunate is that it appears that once the weaker ball joints failed and the tire and hub separated from the suspension, the much stronger suspension wishbones caught the next vertical guardrail support and that seems to have caused the truck to spin 90 degrees to the right (from the occupant's view) - anyone who has seen crash videos can tell you the fastest way to get a 4x4 to roll is to get it moving sideways relative to its direction of travel. It looks like the HMMWV was using soft doors (not X-doors), and those don't have much if any structure when it keeps occupants contained in the protected space. I'm guessing that the unfortunate lady who lost her leg, had it flail out during the initial roll and the vehicle probably came down on her leg during the driver's side slide before it finally fell to its roof.
It pains me to think about her condition, and I hope she pulls through. My thoughts and prayers are with all of them, especially the lady reservist.