What causes endometriosis to develop in rectal area?

Genetics. We are now learning that genes have a significant role in determining who gets endo, and how severe it is. Somewhere between 10-20% of women with endo have bowel involvement. Rectal and sigmoid locations are the most frequent sites for bowel endo, and it causes pain with bms all month long. You usually can't see endo on colonoscopy so many women arent diagnosed by GI docs to whom they're referred.
It can be anywhere. One of the main theories as to the cause of endometriosis is the flow of menstrual fluid through the fallopian tubes at the time of the menses. If that fluid, and those cells, fall into the area of the rectum, the cells can implant and grow in that area.