Here's a theory, however unlikely it may be.
You recently did your waterpump and such, right? You probably removed the radiator to get the space up front to work on the car?
I've found that the 10 year old plastic on the radiator can be quite brittle, and you may have inadvertantly caused two minor hairline fractures when you re-tightened the hose on the radiator.
With heat and pressure, well, POOF!
Considering that it's just coolant circulating in there, in order for it to "blow", it would require excessive pressure, which maybe
a faulty cap could have caused excessive pressure? And the only thing that could cause a pressure increase was heat, but you said the engine didn't overheat at all.
But you'd think that if there was excessive pressure that it would've blown through one of the gaskets (water pump?) or through the upper or lower radiator hose.
Am I making any sense?
VK