Manifold pressure increases with reducing RPM because it is increasing toward ambient air pressure.
When running, a simple piston engine (not super/turbo charged) always has a manifold pressure of less than ambient air pressure. When stopped the manifold pressure is the same as ambient pressure.
The carburettor manifold is a venturi (waisted tube) which has the effect of increasing the air velocity through it whilst reducing its pressure.
This reduction in air pressure draws fuel from the tank through a metering valve.
As a piston engine slows down so does the air travelling through the manifold and thus manifold pressure increases.