I'm with Clutch on this one - the loosening nut is so gradual and the shearing force on the cotter pin so small (assuming the correct size cotter) that it shouldn't shear. I would look at the inner bearing races not being installed all the way back in. They could be working inward every time sideways stress is on the wheel, causing an apparent loosening of the nut. And as Kipper said, they are not supposed to be completely tight, anyway.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.