By the look of the axil, seems like the sheared end has some rust and just a small portion of the metal is a clean break. Good thong you weren't going 65 MPH down the highway at the time.
Did you have any vibration that was speed related in the past?
Can't imagine why it broke there, unless you had a bent wheel or perhaps the wheel mating flange was bent and the cyclic twisting work hardened and finally broke. Not like you had 100% traction and laid 800 pound/foot to it.
