Very basically, it's because you're putting down paint that's too wet. There's too much water pooling on the paper, so as it dries the pigment has time to migrate within the wet area. It moves towards the edges and when the paint dries the color is uneven, darker at the edge. Why it ends up at the edges has got to do with the water moving towards drier areas of paper, but you don't really need to study the physics.
Solve it by reducing the amount of paint you put down at one time. Work with a drier brush, wiping it on a dry cloth before you pick up new paint. Pull some of the water out of your brush before you put it to the paper by placing a cloth at the base of the brush hairs (the ferrule end, not the toe). Don't add extra water to paint already down on the paper. If it's too dark, lift some off with a dry brush or cloth.
Also check the paper you're using. If it's got too much sizing, the paint will sit on the surface rather than absorb readily. It's worth trying a few different brands and weights of watercolor paper; don't just settle for the first you found.
