hard riding over a long time
poor quality wristpin bearing and or worn wristpin bearing
poor quality wristpin steel
slightly mismatches part tolerances
or a combo of all of the above.
i think its bad/worn parts man. For example i have noticed that some cranks come with higher quality (feeling and looking only not other way for me to tell) needle bearings than others. Its been my experience that needle bearings (not necessarily on cranks) tend to fail faster than most other types bearings.