I see bent hangers causing this all the time. You lay your bike down on the drive side, or the bike gets whacked in a self closing door. You don't notice it, and you don't remember it, because it doesn't correlate with the breakage. You ride along unknowingly with a bent hanger. Trouble starts much later, when you shift to low gear. The bent hanger allows the derailleur to travel farther in than before. Bang.