To me , it looks like the spinner attachment bracket , visible in the top picture on the backplate , just below the prop leading edge where the 2 rivet tails are , has failed ! Centrifugal force has tried to pull that part of the spinner outward . Of course it can't move because of the screw at the front bulkhead , so the screw edge has acted as a stress raiser causing the crack to propogate from it , as you can see . From memory , the brackets are steel and are known to fail . If so , exaimine the edge of the break . If its nice and clean and grey/silver , its a fresh break . If it shows a dirty red/brown edge for most of its length with only a small clean grey section , the crack was pre-existing and only failed once at a critical length . Either way , there is nothing you could have done to cause this , tell him to take a hike !