I don't have one! But it has nothing to do with spinning. It could spin virtually for ever with no problems. What kills them is misalignment that causes fatigue.

 

That, along with overheating a torque converter, causing it to possibly balloon forward and forcing the shape of the flexplate back towards the engine, against its contour.
A damaged crankshaft thrust bearing will cause excessive end play of the crankshaft, and can damage the flexplate by causing the back of the plate to scrape on the engine to transmission adapter plate. This description, is from personal experience.
All in all, the two possibilities I've described, are forms of misalignment like Mark said.

 

Add that to tolerance stacking. If you have four parts bolted together with a .005" tolerance and they all are on the outside edge of the tolerance you can easily be .020 off.