Is this type of failure due to the bolts holding the fan on becoming loose, or the fan having an aluminum mounting surface?
What destroys the old style is there is a rubber bushing that sandwiches that stuff all together in that rubber deteriorates and goes bye-bye and then nothing's holding the fan blade and it just sits there and Wobbles around and destroys itself
As Coffey said, the rubber part is supposed to be replaced each time you disassemble the fan, and not reused (per TM). Of course, nobody ever does that. So they languish for a decade or two until they get enough slop to vibrate themselves apart.
I also suspect the problem is exacerbated by worn drivelines that are introducing vibrations from the other end. Harmonic vibration from a driveshaft shakes the engine, and the rubber fan clutch bushing is already loose/weak and can't hold up.
It's worth noting for those that may read this thread in the future that the fans are not interchangable with the clutches. There is an original style clutch (which has threaded bolt holes on the front) that uses the multi-piece fan with replaceable rubber isolators. Then there is the newer style clutch with studs on the front, which the TM directs you to upgrade to when servicing anything that removes the fan (e.g. engine swap, etc.), and it uses the new one-piece fan with steel center hub. Though they look close, the new fan will not fit on the old hub, or vice versa.