I have completely inspected my drivetrain for problems--nothing. Also, I am not a roadie. I ride mountains which are much harder on chains (dirt and short bursts up steep hills). I am good about being aware of my chain configuration. In addition, the way the chain breaks seems to indicate that it simply has been pulled too hard in a linear fashion. Personally, I think that it is the chain's fault as I have gone through two of them on a new bike.
No reason to start an argument about leg pressing, but let me just mention that most people count the weight that they put on the machine and fail to consider the fact that the weight is on a slide. In addition, very few people actually bring the legs all the way into a 90 degree angle. So, on the 45 degree leg press machine that I use, the actual weight on the machine is over 1000 pounds. I doubt there are any 10th graders that have ever done that down to a 90 degree!!