The connecting rods transfer the rotating force of the crank pin to an oscillating force on the wrist pin. Connecting rods are split perpendicular to their centerlines at the crank pin end for assembly of the rod onto the crankshaft.

Fatigue is the main cause of broken connecting rods--especially in older engines. The constant compression during the power stroke and stretching during the exhaust stroke, over thousands of times a minute, eventually wears the metal out and it becomes brittle and finally breaks.

The only solution is an engine rebuild where you pull the rods and replace the bearings. It's likely that the flailing rod damaged the crank journal surface, so you'll definitely need the crank polished and most like turned.