Let's suppose some children were to ram a very large potato into the exhaust pipe of your car. You would probably find it impossible to start the engine because no exhaust gas could escape to atmosphere.. And because the exhaust gas could not get out, little if any mixture could be drawn in. The volumetric efficiency would have reduced to zero. That is an extreme case of course, but it demonstrates the effect.
A less extreme case is that of the wate-gate controlled turbocharger. As altitude increases, the wate gate gradually closes to make the turbine spin faster in order to maintain constant MAP. This makes it harder for the exhaust gas to escape, so the volumetric efficiency and power output gradually decrease.