The plastic at the base of the fan blades fatigues with age. Because the fan is pulling air into the engine compartment the blades are continuously being pulled forward. As they separate from the hub they have a nasty tendency to make contact with the radiator and other parts around the shroud before they separate completely. The loose blades sometimes get caught between the radiator and the remaining blades on the fan for a few revolutions which tears up the radiator too.
2006 Z4 3.0si Sport Package (But I'll still hang with my "homies" in the E36 forum)