The pressure of a gas is caused by

The rapid motion and collisions of molecules with the walls of the container causes pressure.

The rapid motion and collisions of molecules with the walls of the container causes pressure (force on a unit area). Pressure is proportional to the number of molecular collisions and the force of the collisions in a particular area. The more collisions of gas molecules with the walls, the higher the pressure.

(Don't know for sure but ...)