Just to set the scenario for those who are not from Michigan, this happened in possibly the hilliest part of I-75 (although there is another spot about 85 miles north). Based on the comment that it was where there are 3 lanes, there is a third lane for slow traffic up the hills.
Therefore, she may have been coming down one of the hills and gaining speed. Also, there were spotty thunderstorms that moved across the region yesterday, with dramatic temperature drops. There may have been heavy wind gusts.
Most importantly, it appears that the roof-rail airbags deployed on the Avalanche possibly assisting in the minor injuries.