If a small drop of tiny particles such as pollen grains are dropped into a drop of water on a microscope slide, they will appear to vibrate and spread out. The primary reason for this is

The primary reason is that the pollen grains are being struck by water molecules that move in different directions. These strikes fluctuate and sometimes are uneven.

Explanation:

This vibration and spread out of molecules in water is called Brownian Motion. It is the result of the collision of small particles of water with big particles of pollen. As the particles of water move randomly hitting different sides of the pollen particle, at times, there will not be a coordinated movement, but as the movement of water particles is random, there will be moments when one side of the pollen particle will collide with more water particles, when this happens there is an unbalanced force that makes the pollen particle moves in a direction.