Change in volume causes change in pressure
A change in volume of a fixed mass of gas at constant temperature causes a change in pressure of the gas.
The bubbles exhaled by a scuba diver grow as they approach the surface of the ocean.
The pressure exerted by the water decreases nearer the surface, so the volume of the bubbles increases as they rise.
Increasing the volume of a gas means that the walls of the container are further apart.
If the temperature is constant, this means that the gas particles travel at the same speed.
This means that the gas particles will collide with the walls of the container less frequently. This leads to a decrease in pressure.