Wow, that is one heck of a slip on the hitch. I wouldn't attribute it to a new hitch though. We ALL should check the bolts from time to time as it could happen to any hitch.
It would be nice if yours had an offset six sided washer like the hitch pictured above. Does your hitch possibly have an adjustment bolt at the bottom between the stinger and the hitch head?
It just seems wrong to be trusting the head angle to just pressure from the side bolts with absolutely nothing to help prevent that kind of slippage.
On a positive note, at least it will be really easy reset the bolts back to their original positions.