Air is composed of molecules that are not linked to each other and that are freely displaced in space. Air molecules trapped in the pot occupy all of the available space and rebound against the pot walls.
When the temperature increases, the thermal agitation of the molecules increases their diplacement speed. When the water boils, the water vapour increases the quantity of gas in the pot. Because it is hermetically closed, the pressure does not stop increasing. The security handle is released to stabilize the system.