The driving force behind fluid pressure within an open container is gravity. Gravity pulls fluids toward the center of the earth. Because gravitational attraction between two objects is inversely proportional to the distance between the objects, gravity pulls more on things closer to the earth than on things further away. For this reason, atmospheric pressure is lower on top of a mountain than it is in a valley.
Additionally, how deep the fluid is has an effect. If you imagine a very tall column of fluid, and think about gravity pulling downward on every molecule in the column, you will realize that the molecules near the bottom are not only being pulled down by gravity, they are also being pushed down by the molecules above them. Hence the total pressure in the bottom of the column will be higher than the pressure further up, where there is less material pressing down.