the two studs/nuts have been cross threaded.
shit happens.
this might have been caused by you putting them on wrong, or the threads on the studs/nuts not matching, or even being way over tightened.
just be careful next time.
when removing a wheel, i jack it up a tiny bit then crack/loosen the bolts while its on the ground (always doing opposites), then jack it up then undo it all the way.
when putting it back on, (again doing opposites) i tighten the bolts slightly to around 30% torque while the wheel is in the air, then drop the car and do them up all the way.