I'd guess at either the pressure release valve failing in the cap, or the system over pressurising and blowing out of the relief valve.
you could remove the cap (when the engine's cold) and then place you hand over the opening, and see if the cooling system is pressuring, which would point to head gasket problems etc.
Or, replace the cap and see if it happens again!!
I'd want to discount head problems though, if i were in your shoes.