lost a wheel and tire

Traveling through an interstate construction zone often times having the right side wheels on the rumble strip, about six miles down the road pass the construction a trucker pulled alongside and motioned I had a problem. The picture below is what I found. Never heard or felt a thing when the wheel came off. The studs were sheared straight and even with the brake drum. That occurred in 2008, in 2014 found one stud broke all the rest were fine, and again in 2015 had two break. Like the OP I’m at a complete loss as to why, I always use a torque wrench and check the lug nuts frequently. Thinking the drum may have a crack or other defect I replaced it after the last two broke.
With the drum laying flat, holding a heavy duty center punch with vice grips, a two pound ball peen hammer and a good solid blow, the broke studs will pop right out.