... my thoughts are that a line break is usually extenuating circumstances such as impact damage, rubbing against something, or IMO majority of the time overstress from higher than normal loads.
Interestingly, all the line breaks that I've had are all in the same place, right before the sheath. My lines were usually a year old and I've overloaded them, snap and swim. Brand new they may have had a breaking strain of 400kg but over time, maybe its only 200kg???? Pure guess here.
I had all 3 front line snap in mid kiteloop while I had my back to the kite and kite was on its way up, then bang and splash, swim again. Its almost as if the 5%(guess) stretch or spring if you will, can handle it over the entire length but at the points where it can't (the stitched sheath end) stretch its alotted 5% is beyond what it can handle.
I reckon when you run your lines out and its windy, the ends seem to knot up and with kite fever, you pull and yank the ends abit, putting stress on it and with no re-enforcing, it wears the line out just that little bit more each time and right at point it can snap due to its lower ability to stretch, maybe on a micro level you are cutting tiny fibers each time weakening it.
Anyway, what ever, just a few thoughts.
cheers,
Robbie
Full time work, oh no, does that mean less time for board building??? 
But, finishing one as we type this:) Scratches resistant one!
