It is a freezing point depression issue. In fact after about 40% H20 (I think it could be closer to 50%) the freezing point actually does begin to go up.
Think of the salt in ice water example. you add salt the FP goes down. Now in your example it is simply water in ethylene glycol. Hope that help!
Oh and when you reach that magic composition, all of the sudden you can think of it as antifreeze diluting your water.
The true explanation comes out of the math derived by the Clausius-Clapyron equation. That is way beyond the scope of what you need to know. A phase diagram sufficiently explains the phenomenon.
