What are the causes of astigmatism?

Refractive Error: Here is a great article by mayo clinic: http://www.Mayoclinic.Com/health/astigmatism/ds00230/dsection=causes. Astigmatism has to do with distortions of the curves in the cornea or the lens of the eye.

Cornea, lens: In astigmatism, light that enters the eye is refracted more in one direction than the other, allowing only part of the object to be in focus at one time. 95% of the time, this is caused by differences in curvature in different axes of the cornea on the front of the eye. The cornea does 2/3 of the focusing of light in the eye. The other 1/3 is done by the lens, which causes 5% of astigmatism.