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