Higher grade bolts are heat treated medium carbon steel, they are supposed to have enough of the brittle characteristics negated by a tempering secondary heat treat. I have found through experience, grade 5s to be the strongest balance in industrial service. I suspect your high grade failed bolt had an improper heat treat during manufacture. It goes with the tree must bend with the wind in order to not break. A grade 5 will stretch and deform before failure, a grade 10 fails all at once, due to its designed inherent lack of ductility..
