The most common and main cause of a person developing Astigmatism is because of an error in the shape of a persons cornea.
When a person has Astigmatism the lens of the eye or the cornea "which is the front surface of the eye" has an irregular curve to it.
When that happens it can change the way that light passes, or refracts, to your retina which is what causes the blurry vision or fuzzy or distorted vision.
That is what causes Astigmatism.
Most people who have Astigmatism are prescribed either contact lenses or eyeglasses to naturally correct and fix the Astigmatism but sometimes a person with Astigmatism may have eye surgery done to fix the Astigmatism and restore the persons natural vision.