Causes of Big End Failure
 

Been asked this question alot over the years, so thought I would post up some info, some known some sourced from the interweb.....

Main reasons being :
Dirty Oil - Dirt wears bearings
Oil Starvation - Lack of lubrication
Crankshaft Oil Seal - Resulting in oil starvation / lack of oil pressure within crank
Detonation - Causes piston/rod to force against the film of oil protecting the bearings
Spun bearing - Caused by lack of lubrication and heavy contact between rod and bearing. This can cause friction welding and hence bearing breaks free and spins.

Some light reading, also covering assembly errors and tolerances : http://www.nb-cofrisa.com/docs/web_fallos_ing.PDF

More Info :
Another cause of Big-End failure could be due to overheating.
This is because the bearing material gets eaten away by the very acidic coolant/oil mixture & because of this, they are compromised to a very short life span, even after you change head gaskets & everything seems okay at first.

Causes of oil starvation:

With the flat bottomed oil pans (WRX) oil can move away from the pickup causing starvation.
Broken oil pickup pipe
Clogged gauze filter in oil pickup
Oil being pulled out of the top of the sump by acceleration/cornering/braking onto the crank, and returning too slowly

Very interesting read, good explanations, never really known what a spun bearing was.