You might want to check your frame for cracks trekkie. A good bike will have parts that will give in way before the frame will. I'm not saying that your 820 is a depot store bike, but the parts spec is very close to that. Plus you have to think how old that chain was(It mayhave looked new, but the person could keep the chain looking new.) Or it was a defective chain, no way to control that, and what psi the dirt jumper was running could have been low. You got to put all of those equations together before assuming something like that.

Back to the topic, do what maelstrom said, check bolts before and after each ride.