I think it was a catalogue of problems, the perfect storm if you like.
Guess 1:
Say the pump stalled and was generating heat of it's own in to the water. The water isn't flowing any more because of the pump so the radiator isn't cooling it down. As the heat increases so does the pressure (adding certain additives will raise the boiling temperature, ie engine coolant above 120C). That pressure has got to be released somewhere; in your case the weak part of the tubing. The warmer the tubing gets the softer it becomes.
Guess 2:
Talking of tubing, I don't like how your tubing hangs off the cpu block. Looks like it's collapsed slightly and no longer "full bore". That in it's self could have caused your weak point. The rupture looks to be in the area where the tubing has folded/compressed. When the tubing became warmer and softer, perhaps different parts of the tubing changed tolerances at different rates. I'd suggest replacing your tubing if you can as it may have been stressed in other places and address the issue of the way the tubing hangs off the cpu.
Just my speculation of course!
