Shower head valve

I'm not a physicist, but I suspect the problem comes from the fact when you shut off the sprayer valve, you still have both hot and cold valves open. The water heater just got a shot of cold water in it so the fire's come back on. The cold water in the heater begins to expand pushing hot water past the shower valve and having nowhere else to go, it pushes the cold water back down the plumbing. I know it is a closed system however, RV hot water tanks are made with a 'air pocket' at the top to accommodate expansion of the water and with the shower sprayer configuration, I suspect water can easily 'move around' in the plumbing with the cold water at the shower being pushed back towards the cold inlet at the water heater filling a substantial part of the cold water line with hot water, scalding you when you open the shower spray valve again. I, too, have noticed this phenomenon and this was the best reason I could come up with. Anyone else care to post a better explanation?