how old were the bolts? Very possible they had been over torqued in the past which would lead the bolt to enter the necking region and making it impossible to torque properly.
I had this happen last year with wheel bolts on my Volvo (they use bolts, not studs). Had the originals on there for 13 years. 225k miles, tire rotations every 5k when I changed the oil. A few times, shops did it and would over torque them. Manual called for 81 ft-lbs, A few times I had to use a 3' breaker to free them up. Then, last fall, my wife was driving into town and called to tell me the wheel fell off. I thought she was smoking something and told her to have AAA tow the car back to the house. Sure enough, 4 of the 5 bolts snapped. Looking at the remains of one of them, it appeared that one had failed and after that, the others followed rapidly. I then replaced every bolt on the car with new. I was glad to be able to put my root cause failure analysis class to work. It is amazing what you can tell from looking at cracks, and how they form, and how it tells you what happened.
