In any garage, if a bolt/nut is too tight when pulling at it, the rattle gun is always used to loosen it. and it could be argued that the rattling effect when loosening is a lot less Brutal than the contstant torque forced on the nut with a Strong bar.....
How many of us have stood or seen someone stand on a wheel wrench or Bar to crack open a Nut?????. You may bet that 16 to 20 stone standinf on a lever will produce more force on a Nut than a Rattle gun... and the Nuts dont break like this..
If it happend when tightening it then maybe the torque was set too high, or the guy had it on wrong thread and we could blame him and call him a fool..... but then again regardless it should strip threads before the Nut would break like that...
I dont think the Chap is in anyway at fault, i would have opened them all with the rattle gun myself, as do any tyre garages/ Garages ive been in.
Ive taken off nuts off cars that have been dogged, they have brought threads off the studs with them, stripped the nut of threads, even broke the stud, I have never seen a Nut come a part like that before..
I reckon it could have originally been forced on the wrong threads, and as a result whether it was by hand or airgun.. this is how the nut sh*t itself.. = poor quality nut imo..
Or
There was a flaw/ crack in the nut from manufacture or from being taken on and off over a period a time and it broke here... but again = poor quality nut imo.
Then again, if there steel and it says dont use Airgun... then they cant be good.