The location of the breakage can be a strong clue as to the problem. If the break is towards the front of the crankshaft the vibration dampner is a strong suspect. If it is a rubber mounted dampner the rubber can allow the outer ring to slip and defeat the purpose of the dampner. Crankshaft breakage towards the front of the shaft is usually the result. If the breakage is towards the rear, I would inspect the flywheel and/or clutch for being out of balance or the torque convertor, if it has one. If the engine powers front driven accessories they could also be a factor.
Hope this helps...Good Luck.