Normal wear and tear, defective parts, rough shifting, who knows. Check the clutch faq http://www.myturbodiesel.com/1000q/clutchfaq.htm there are more details there. I've broken a clutch fork at 180,000 miles on a daily driver so I always replace it if the clutch has high miles and the transmission is coming out.
Maybe both? I would think that on one hand a stronger clutch would stress it more but on the other hand it's cheaper to make one part that fits all VW engines, and since the TDI isn't the strongest, that it would be well within the limit.
I know that some people make stronger clutch components, longer clutch rods, stronger clutch fork pivot balls, etc., but I suspect that there aren't enough people at very high power levels long enough for this to start showing up and I don't think there's a market for them.
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