Condolences, dude. What a pisser.
Anyway, the first possible cause that comes to mind is maybe a defective piston/ring/con rod/pin. These parts can have tiny defects and can fly apart without any notice, even though you have done all the proper maintenance.
Another possibility: broken timing belt allowed valves and pistons to meet violently, causing destruction of a piston. This theory is less likely, though, because the Zetec may be a non-interference engine.
