The tyre shop I use have those rated torque extensions for their rattle guns. A couple of them shattered after prolonged use. They have started using a normal extension to get the nuts tight then swapped to the rated extension. I have had no problems, but I usually check the nuts with a torque wrench at home.
Is the tyre shop the regular one you use? If it is, and they are over tightening the wheel nuts they may be weakening the studs.