What causes cylinder shafts to snap?

Is it the constant switch hitting? Or is it just the weight of the car, even if you just drive cool, that causes the rear cylinder shaft to snap?

Reason I ask is I'm now scared to drive long distances since I snapped a cylinder. I want to drive to NOPI in Sept (10 hr drive) and I'm just wondering if it'll be safe if I just don't play with the suspension. You know, just drive cool the whole way, maybe clown 1 or 2 foos along the way.