I have a theory/ idea of why?
A few days before I changed my air filter lid seal. And when this happened on the freeway, I noticed as soon as I popped the bonnet up the hose that runs from the top of the radiator to the top pressure tank on the LHS move. My only theory is that I had somehow folded or wedged this hose and blocked of the hose. There for not letting the radiator and cooling system relive the excess pressure to the top tank. The pressure was so great in the cooling system, the bottom hose clamp, which is the OEM spring loaded clamp type, opened up momentarily, the excess pressure was relieved( hence the coolant all through the LHS wheel arch) and then simply closed up again once the pressure was gone.
I was bloody lucky I didn't bust anything.