Nut backed off ball caused accident

I am thinking what may have happened is that the OP may have over tightened the nut, causing the lock washer under it to spread and break. After a little while on the road, leverage on the ball would have caused a piece of lock washer to spit out, leaving the nut very loose on the ball's shank. The main point I am making is that even if you use a torque wrench, have a good look at the lock washer after. I've sure seen a lot of them spread and try to come out from under the nut.