The two causes of astigmatism are: a distortion of the lens and a distortion of the cornea. So for astigmatism to develop a refractive error needs to occur. Meaning for a person to have astigmatism the light needs to fall into the eye incorrectly. Changes in the eye cause the light to focus or merge before or behind the retina. And in the case of astigmatism that change is either a lens distortion (lenticular astigmatism) or a cornea distortion (corneal astigmatism). That's why astigmatism causes objects to appear blurry. Because the focal point is not on the retina, as it should be. The light rays focus and merge too early or too late. So that two visual stimuli are happening on the retina, where only one merged stimulus should appear.