you said you had spacers on your car, right? were they hub centric ones? If not, that spacer might have had plenty to do with it. Basically, the studs were absorbing a lot of the stress cuz of that spacer.
Take a look at what happened to this guy:
http://www.planetsoarer.com/offset/studs.htm
In any case, you should be able to complain to them about this. They were the last people that worked on your tires and wheels.