Yeah, I would suspect that the hub was nowhere near tight enough on the taper and/or it had poor fitment on the tapered surfaces.
As Brian noted, this causes the key to drive the hub and not the taper surface - which can cause a break just like this. Also, looks like it was cracked for a while before it finally snapped.
Torque those hubs boys - to 205 lbs . . . and if you need a bigger Torque wrench, go buy one!
(that cheap bend-beam bar torque wrench that most of us "grew up with" won't cut it!)