Good question! I have seen many a marker spread and bleed through paper, usually to my dismay. To explain how markers spread on dry and wet paper, we need to understand how markers work, and what paper is.

A marker or a pen is a tool used to spread ink around. Ink is usually a liquid solution with pigment and other things that make the pigment stay suspended in the solution and stick to paper.

The paper we use today is usually made out of pressed wood pulp. If you were to look at a sheet of paper under a microscope, you would see thousands upon thousands of cellulose fibers heaped and tangled all around each other. These fibers are like long, spongey ropes.

When you write with a marker on paper, you are dumping ink all over these cellulose fibers, which in turn soak up the ink. If you add too much ink to the surface of the paper, the ink flows away from your marker and the saturated cellulose fibers to the closest fibers that have absorbed little to no ink. That's what you see when you see a marker spread on paper or bleed through paper. As the water or alcohol in the ink dries (evaporates), only the pigment is left behind. Markers spread easier on wet paper because most of the cellulose fibers in the paper are already saturated with water. The ink has to travel a ways to be absorbed or it has to stand on the surface of the soaked paper. Markers spread on paper for the same reasons water spreads on your clothes when you spill a drink.