This is normal, when you turn off the engine when hot, the water still absorbs the heat from the engine, as it is not being circulated through the radiator it continues to get hotter and hotter, this will form air bubbles, and hot spots of water in the system, it's merely this finding it's way into the expansion tank, whereupon it meets cooler water, and air space and so the air comes out of the water, it couldn't before as there was no free space, as water is much denser than air.
Whenever water is spun it causes air bubbles as well, this is why ship propellers erode, due to cavitation caused by spinning propellors, it causes pitting of the blades.
tabetha