What causes a harmonic balancer to fly out of car while driving?

The balancer is actually two separate pieces, the inner hub that bolts to the crank, and an outer ring. These two pieces are 'glued' together with a flexible glue. The way it works is the hub turns with the engine, and any engine vibrations are absorbed by the outer ring. The flexible glue does degrade after a while, or maybe the engine is vibrating and the balanced just couldn't keep-up any more. Got a new one.