I bought a quad with the exact same problem. Don't know how they managed to break it, but my point is it's not the first one to ever break a weld there. I've beat mine in the rocks relentlessly and never broke one with a PRM skid and now the stocker. Maybe it just takes an unlucky hit like Boomer said.
Throughout the second lap I would feel an occasional jerk. I knew something was wrong but kept pushing it, by the start of the third lap it I could hear something rubbing bad, pulled over and one of the bolts was gone and the others were loose. I gimped it back to the truck just fine!
I'm not looking forward to getting the old sprocket hub off...