There are various thoughts on the cause of endometriosis with some research to back them up but not enough to support just one, which is why there's quite a few current thoughts.
One main thought has been mentioned in the Endometriosis UK book:
Endometriosis: The Experts’ Guide to Treat, Manage and Live Well with Your Symptoms
They mention that endometrium cells have been found in the foetus as the uterus is developing, and that sometimes during development / childbirth a "period" is experienced - newborn girls can have slight spotting and that has been recorded statistically. This "period" is due to the increased hormones and stress hormones during birth / entering the world, which is thought as there's a plug of mucus in the cervix some of these cells can embed outside the uterus causing the start of endometriosis. Endometrium like cells outside the uterus have been found in female foetus' which supports a theory that some women are "born" with these cells already outside the uterus. This bit in the book more focuses on the research of what the cause of some pre-puberty girls having the cells already present but not active. It does relate it to low white blood cell count (not a mutation) which is why I thought it might relate to the above you've mentioned and could in the future provide early detection - at present blood test detection just shows the possibility.
It's an interesting read, including the whole book!
