What is diffusion?  Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration due to their kinetic energy.  Example:  you open a bottle of perfume, and set it on the table; soon you smell the perfume molecules that have diffused out of the bottle.

What causes diffusion?  Kinetic energy causes the random motion of molecules.  As molecules move at random they collide with one and other.  The chance of a collision is less for those molecules that happen to be moving up and away from the bottle than for those that happen to be moving toward the bottle.  Thus the molecules tend to diffuse.