radiator overflow sends out coolant after engine shutoff

[dmjlambert]

I have a stock setup of 350 engine, original 4 row radiator, fan with clutch and shroud. I top off the radiator and put the cap on, and take the truck for a drive for 30 minutes. When I get back, after I shut off the engine, coolant comes out of the overflow tube and forms a puddle under the truck.

I guess I was expecting any overflow due to expansion would have occurred on the road, not after I shut the engine off. Is this because the engine gets hotter after I shut it off, compared to when it is running? I just thought it was kind of funny.