There's several reasons, rust on the hub contact points, the studs settling into the hub, excessive vibration, etc.
The wheel you lost is whats concerning me. Ok, you installed the wheels without a torque wrench. Either they were all left loose which beat the studs out or a little too tight, one, then another popped, then all of them went.
I'd be wary of the studs fatiguing and giving out. Any 1/2" stud should be good to 100 pounds easily, if they keep coming loose and needing tightened up the stud is likely stretching and about to give out.
Probably not cheap but I'd give thought to replacing the remaining studs, just in case.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10