Originally Posted by
Retro Grouch
It's caused by failure to follow directions.
Check out the torque spec that's engraved on the lockring. 40 nm = 30 lb/ft. That's a good amount. Imagine a 30 lb weight hanging off the end of a foot long wrench. It's certainly more than I would use if I were depending on my own judgement. If you tighten your cassette that amount, it will lock all of the cogs together so that individual cogs can't bite into the freehub body.
To be honest, every time I torqued a lockring to the spec'd amount it came loose a couple hundred miles later. I now use the "torque until it feels right" method.
And 30 lb-ft is not much at all.