These bolts were tightened by hand and failed at an unreasonably low torque. I suspect that this was due to cost cutting on the part of the bolt manufacturer.
It appears that they can save about 8% on the cost of fastener production by not heat treating the bolts. This would enable the bolt vendor to under bid the competition by selling bolts below the cost of making properly heat treated bolts while still making a profit.
This is the second time I have seen bad bolts in recent years, and I hope that manufacturers are testing the strength of bolts they order when they are used in critical applications where failure could mean injury or death.
This site also has some additional information about bolt failures.