crankshaft keyway

[Spro]

What would cause a keyway to shear off a crankshaft? I had my timing chains replaced due to them being noisy and within 8 months/9000 miles later my truck suddenly dies. In the end the keyway sheared off the crankshaft. The valves in the heads were still in place but bent as I figured they would be. The dealer at first told me a valve dropped out of a head causing the keyway to shear. Won't happen if you ask me!!! They did something wrong didn't they? Something internal failed and it was due to their job they did. Pully not torqued properly or did a chain fail? They should be responsable but won't take responsability.