What are two things that molecules can do to cause an increase pressure?
They can move faster, or slower.
"Pressure" is the result of molecular movement - or compression by some other outside force for solids and incompressible fluids.
On a molecular level, even apparent changes in volume (expansion) are due to molecular movement. Molecular movement is usually affected by temperature. Increasing the temperature increases the kinetic energy of the molecule, expressed by more rapid, or longer vibratory movements. It is this increase kinetic energy that is expressed as observed "pressure" in gas and liquid systems.