I am not what I would class as an "aggressive" cyclist - either in terms of riding through traffic, or in the way I actually ride my bike, but I did manage to break a chain once. As a previous poster said, trying to start off up a steep hill in too high a gear, I applied plenty of weight welly to one pedal and bang went the chain. Luckily it was only about half a mile walk to home. I could have understood breaking the chain if I had been a really strong rider, doing a flat out sprint or something, but not someone like me.
I decided to replace the chain, but before chucking it away I thought I would take a look at it, just out of interest. When I cleaned it and examined it, lots of the little flat plates in the chain had cracks spreading out from the pivot points in them so clearly it was only going to be a matter of time before one of them finally let go.....
Once in a while, clean your chain thoroughly and examine it closely. It might save you a long walk home.....