The nuts when torqued on their first use are deformed by a predetermined amount (the torque setting given). When they are removed they do not return to their virgin/un-torqued state. If used again and tighted even to the same torque setting (adding the same degree of deformation) in reality there is a compound deformation (of the two attempts) which deforms them tighter then the design requires and beyond the force required to shear the bolt (don't know about you but I can't recall ever seeing a nut strip the threads its always the bolt that gives).....
