"What causes conrod bushing damage?"

1-Using a starter to force an engine to start will sooner or later cause conrod damages. On a flooded engine it is even worse. Thats probably the main reason for a conrod bushing damages /failure. It is better to find out why my engine needs a starter in the first place.
2- The lack of lubrication "lean runs" is another reason for conrod damage and I also think castor oil is needed for superior lubrication in that area.
3-Wrong tolerances "usually too tight" between crankpin and big conrod end is another reason for bushings damages