Almost looks like a situation where the two mating parts of the axle hub were held together by just an upper and a lower bolt and the lower bolt was missing. Such that the weight of the tractor put huge leverage on the bolt in question to both bend and tension it as the mating parts started to come apart. Obviously, if this was the case and other bolts were missing, the hub wold have completely fallen apart when this bolt broke. Maybe an error like this occurred during assembly and the worker just "hid" it? Puzzling!
If I were in OPs shoes, not only would the dealer be TOTALLY fixing this, but also thoroughly inspecting (and torque checking) a LOT of other fasteners, and I'd be asking for additional extended warranty at no cost.
If I were in OPs shoes, not only would the dealer be TOTALLY fixing this, but also thoroughly inspecting (and torque checking) a LOT of other fasteners, and I'd be asking for additional extended warranty at no cost.