Originally Posted by Hondo6

From looking at the photos on this eBay listing for a SP-7410 (they're much better than the SP-7410 photos on Velobase), it appears that the SP-7410 post assembly was constructed from two different pieces. The post "head" appears to have been swaged onto the post tube. The upper and lower rail clamp plates were held to the head by a bolt that passed through the post head and the lower plate, and which threaded into the upper plate. Shimano's EV for the SP-7410 can be found here.

My guess is that the swaged joint between the post's head and tube failed, becoming loose and allowing the head to turn on the tube. The Thing from Fantastic 4 going "clobbering time" and twisting the hell out of the seat while clamped into the frame a few times (or something else that put a lot of torque on the seat while the post was firmly clamped stationary) - coupled with a somewhat marginal joint from day 1 - might possibly have been enough to do that.

But I'm just guessing. Who knows?