Tornado and the Case of the Exploding Propeller

What happens when a propeller randomly explodes for no apparent reason? I found out while flying Tornado and a passing cyclist stopped to socialise. I tried to show off a bit, but as I turned to make a speed run... boom! Splash one quadcopter. The exploding prop also took out the plastic side panel, plus part of the camera mount from the opposite side! We executed a controlled emergency landing and came to rest gently in the long grass.