You're really lucky (if that can be said) that this chain didn't fling up and wreck some serious shit on the bike, or your leg.
I had a buddy that got his leg ripped pretty good from a chain break at speed (He doesn't wear shorts anymore when riding... fool).
If it was the stock chain, I find that most reputable aftermarket (DID, RK, etc.) chains have much higher tensile strength than the stock units. As for maintenance, this chain may have appeared maintained, but I may have been stretched and just re-adjusted to it's max tolerable limit.
Either way, gearing change sounds like a good plan now!