What causes a cow to prolapse?

It depends on what they have prolapsed, since there are three types of prolapses: rectal, uterine, and vaginal.

If it is a female that has a uterine prolapse this can be a result of a birthing complication, and may be due to a chemical imbalance that is telling her to continue pushing, or because of stress on the birthing canal. This can also be true for a vaginal prolapse.

Rectal prolapses can result from either constipation or diarrhea that the cow is constantly trying to push out.. This also can be a result of a high amount of stress, for example if the cow is down and cannot get back up she may put pressure on the rectum until it prolapses.