It says that the marker ink is probably water soluble. The water-soluble markers are often non-toxic and the color is washable, thus can easily be diluted.
When it is on a paper and comes in touch with water it may occur a watercolor like effect. What happened can be compare to a chromatography experiment, where the colors migrate with the water (mobile phase) through the paper and separate from each other. The chromatography separates the different pigments on the magic marker.