What Happened Exploding Radiator?

Idling in mall yesterday, after having been driving maybe 3+ hours,and steam started coming from underneath hood. Added water (that thankfully i hadn't removed from trunk since last summer) and drove the 15 minutes home. This morning upon inspection I found a burst radiator (picture) What could have happened. I flushed and refill to specs two weeks ago.(flush tee on heater hose and disconnected bottom radiator hose for drain.Everything been alright,couple of hundred miles since flush. Preparing to put new radiator in ???? wondering Wha' happened. Could a stuck thermostat cause this? Would like to know before installing new radiator.

Could have been the radiator cap not releasing at the right pressure, but I bet it was just the 17 year old plastic getting brittle and finally giving up. If you can afford it, an all metal radiator might be a better choice for a replacement.

Radiator is two years old and has a warranted that I cannot find.Also can't remember which Advance Auto it came from.

x2 on the cap.

X 3 on the cap,

... and crappy plastic header tanks.

Replacing cap.