Defects aside there are a few reasons why it might have come apart. The balancer shook itself apart from torsional vibration harmonics beyond what it was designed for or an out of balance condition in the engine. Drive belt or belts for the water pump, and alternator if you are running one, are too tight causing the pulley grooved outer ring to slip and eventually work itself off. A crank bolt through the hub wouldn't stop that from happening. It only keeps the hub assembly from backing off the crank snout.
There are one piece balancers that can be run on these engines. Some have a belt groove and some do not and require a bolt on pulley. I'm sure one of the racers like Twisted6 can give you more exact info on these.
