Answer
The sudden drop in pressure can cause superheated water in the radiator to boil violently and spray out at you.
Work Step by Step
The sealed radiator, with fixed volume and contents, is under high pressure because of the high temperature. From Figure 13-22, we see that the boiling point of water increases at higher pressure. So, the water is in a liquid state even though the temperature is much higher than the normal $100^{\circ}C$.
If you are unwise enough to open the radiator cap, the sudden drop in pressure causes the superheated water to boil, and the emitted steam and expelled water can burn you.
