I've been biting my tongue but it has to be said - have you tried putting your finger in it?
Ok, sounds like a pretty good theory. The ECU pulls boost at higher elevation by the pressure compensation table. It is independent of whatever map you are running. It does this to keep the turbo in the area of the compressor map it wants to use (it has to spin faster to pump the thinner air up to the given boost level). Also, you have greater knock resistance at higher elevation I believe because the air is less dense. I know you can run somewhat lower octane safely at least. So you drive down a long ways and the learned values from high elevation are no longer applicable (because your hitting higher boost and your pushing denser air leaning you out), so the DAM goes down while it trys to sort things out.