As we know that outflow valve normally closes during climb and cabin altitude also increase but why does outflow valve open if we have selected manual pressurisation and increasing cabin altitude !
If you're selecting increasing cabin altitude, you are lowering cabin pressure. To do that you have to let some air out of the balloon (the pressure hull). So the valve has to be opened more (they usually never fully close because there is always more inflow than needed to maintain cabin pressure with both engines running - how much open depending on how leaky the pressure hull is).
In normal operation, the Outflow Valve does not generally completely close except maybe for system self test or validation test. It maintains some flow so as to achieve ventilation of the cabin.
The basic system of operation of outflow valves is to restrict the airflow through the valve so as to maintain an 'artificially' higher pressure of air in the pressurized parts of the aircraft. High pressure air is typically pumped into the cabin by the pneumatic system from engine bleeds after some processing to make the pressure and temperature reasonable for use. The difference between the free air pressure outside the aircraft and pressure inside the cabin is called Differential Pressure and is typically around 7 to 8 psi at cruising levels. This translates into a cabin altitude of 6 to 8 thousand feet in terms of the Std Atmosphere.
In fact, a closed outflow valve, in many aircraft, denotes a major air leak due to a rupture somewhere in the pressurized part of the cabin and is a sign of structural damage.
System diagrams displaying the actual valve position may show it as almost closed for most of the flight during operation in Automatic Mode. In manual mode, the pilot has to adjust the valve position immediately upon selecting 'Manual' to avoid an uncomfortable change in cabin altitude.