Looking at the pic, and the fact I have never ever seen a nut break like that.. I'd would think it was either a fault in nut, possible a crack, and it's where the cone starts.
It's possible the nut was drove in on wrong threads last time, but the threads look ok on the stud but the ones inside nut could be a mess.. They could have been over tighted before etc etc or just that nut had a weakness either from mis use or flaw..
But it would of broke regardless of gun or bar IMO, I don't really think there is a wrong way to take off a nut tbh...
I would be having a good look at the rest of the nuts aswell... That's not a hood sign... Never ever seen that happen