Turbo back pressure when using iMAP/eMap at altitude

So i have been researching the impacts of altitude on turbo back pressure.

I am building an boosted aircraft engine so Altitude does in fact impact its VE.

What i have not been able to understand is why does back pressure build as you go up in altitude?

Is it because the air pressure reduces and so the waste gate is closed the entire time so the back pressure rises?

I know that the exhaust pressure rises significantly with altitude but I can't rationalise why?

If anyone can explain it to me please let me know.

The only logical explanation that comes to my mind is that air density (lower ambient temperature) becames higher much faster than pressure drops... That would result in bigger air mass getting into combustion chamber.

Compressor inlet density and pressure drops. To maintain a target absolute inlet pressure you need more shaft work driving the compressor harder at a higher pressure ratio. The only way to do that is lift the pressure ratio across the turbine. Both the turbine and compressor are significantly less than 100% efficient so EMP rises faster.

Why would it? First of all, if compressor efficiency drops it would boost hotter air up until air mass gets back to the target and once the air mass is the same as before we wouldn't be getting more exhaust gases since we keep getting the same amount of air and fuel molecules (well, maybe a little bit more fuel since the air is hotter because of lesser compressor adiabatic efficiency)... Secondly, if the inlet pressure drops so does the pressure outside of outlet making it easier for exhaust gases to evacuate combustion chamber...

The only thing that can affect on exhaust gases pressure is the air/fuel mixture temperature - it should be higher than at normal altitude because of hotter compressed air. But again- the difference should not be that big...

Thank you for this. It gave me a different perspective.