When the coolant (in this case it's mostly tap water) temperature rises it can boil and pressure can grow. When the pressure is exceedingly high that usually results in a pipe bursting or maybe even the coolant cap flying off especially if it isn't on properly.. In this case you can see how high the pressure is when the coolant cap is released.

Yeah so when I was at that company when these buses used to overheat we would bring them into the yard and open the cap and let the hot water spew out and replace the hot water with cold water from the hose. Not a great solution but it brings the engine temperature down a dozen degrees. Of course half an hour later after very gentle driving and no kickdown you get the dreaded red warning light again.