Thermosyphon? Long story short...hot boiling water rises because it's lighter. It has more entrained air (bubbles). When it's heated it expands. As it it cooled by the air forced through the radiator, the cooler, denser, heavier water falls to the bottom. The denser cooler water helps force the lighter, hotter water out.
Raising the pressure of the water, raises it's boiling point. Many older tractors run to hot because people put the wrong cap on them...4# when it should have 0# or 7# when it should have 4#, etc.