Torn water pump pulley

[Hotelbldr]

I had the strangest thing happen on the way home tonite. But I'll digress just a bit . over the weekend I installed a Classic Auto Air perfect fit AC in my 66 convertible. Part of the job requires changing the fan blade to a 35 degree 6 blade fan. As the fan on the car was a 6 blade but not the 35 degree I went ahead and changed it. Not a big deal, the radiator was out already, it just means pulling 4 bolts, pull off the old fan, slip on the new fan, and tighten up the bolts. I have it all ready to drive take it out for a test drive around town everything checks out OK. Today 1/2 way home (15 miles) I hear a funny noise and shortly thereafter the temp guage is heading north. I find a place to pull over (its pouring cats and dogs), pop the lid and to my amazement the fan pulley looks like a stryofoam cup with the bottom ripped off, its just laying there over the water pump shaft. The spacer and the fan are still attached and the fan just spins (like I think it should). What the heck happened? Did I put something on wrong or was this just a coincidence of a part ready to fail?

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66 burgandy convertible; 306 (289 bored .03 and 302 crank) edelbrock: Performer intake,4v carb; hurst 4 spd; 2.8:1 rear;