Why is the radiator blowing after the engine is shut off?

That is common on many cars. While you are driving the airflow from that motion cools the engine coolant. However, when you stop, the air is no longer flowing and the vehicle enters into a phase of heat soaking where all of the heat in the engine tries to cool down and it gets the under-hood area very warm. It can get hot enough that the coolant pressures can damage parts. So, some manufacturers set up their fans so that if the coolant temperature is high the fans come on and continue to run for a few minutes. During that time the engine heat continues to circulate the coolant in a process known as thermosiphon exchange. In that the heat of the coolant causes it to rise, then it passes out into the radiator and starts to cool, as it cools the coolant slowly falls and pulls more hot coolant in. Once the temperature falls enough the fans will shut off.

Now if it was running hot it would still do this but perhaps longer than normal. I would check the coolant level and if it has not been serviced in a while it might be a good idea to do that as well.

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars