I'm not a professional on metal fatigue but I think the shaft has been cracked for many a moon. The 'smooth' part of the crack is when it let go. The 'pebble' finish part is a very old crack. I suspect the noise you heard was the shaft being out of round. It must of started to wobble but only for a few klicks before it snapped. End shaft pressure must have kept it intact until some transient high torsional load snapped it. Thanks for sharing this with us.