What causes the cornea to change shape in astigmatism?

Question

I was diagnosed with astigmatism last summer, I've never had any noticeable vision problems before (I'm 20 years old). The shift from fine to terrible happened quite fast. I've visited an eye doctor, and found out that I've got astigmatism.

So naturally I've tried to google what astigmatism is, and what causes it. From what I've found, I understand that astigmatism is caused either by an irregularly shaped cornea or a lens, but cornea related problems seem to be more frequent.

So I was wondering why does this happen? What causes the cornea to change shape so rapidly?

IMPORTANT EDIT:

As it turned out, in my case astigmatism was a symptom of a disease called Keratoconus, so if you're experiencing rapid change in your vision, and your optometrist can't tell you why, go to another one, or better go to a clinic that specialises on treating Keratoconus, as it is very important to get a proper diagnosis as soon as possible, I've wasted too much time, going to unqualified optometrists.

Answer 1

The specific cause of astigmatism is unknown. It can be hereditary and is usually present from birth. It can decrease or increase over time. - American Optometric Association

You might've had it since birth but without noticeable defect and has progressed rapidly enough to produce a defect in your vision during your early 20's.

Other factors that would affect the development and progression of astigmatism would be ethnicity, the presence of myopia, axis, and subtype of astigmatism. - Investigative Ophthalmology and Visual Science Journals