I worked as a tire man at a service station for a little over 5 years, and only saw the studs shear off on a wheel once. Even after i gave the customer my customary 5 minute speech on how important it was to come back to have the lugs re-torqued, only about half of them came back after 50-100 miles. The guy that came back with the sheared studs didnt come back for a re-torque
Ive had several lugs make 1 to almost 2 complete turns to be re-torqued to the proper spec after being driven for 100 miles...im betting this was a contributing factor to your failure, though it may just have been tired studs.
I always re-torque my own lugs twice, once after 100 miles and again after another 100. The first time most every lug has lost at least 20 ft-lb, the second time they are either fine or up to about 5 ft-lb off.
Just my .02, but re-torque those lugs after 100 miles!!