This is a very good blog that rounds up endo research. This latest article is on how endo tissue is influenced. endocomprehensive.blogspot....
It mentions Mullerian theory. It is believed that when we are a developing foetus some cells, the Mullerian cells, grow in the wrong place in our peritonium. These cells are meant to grow in the uterus. They have the same characteristics as the endometrium and when we have too much oestrogen they activate and become endometriosis. This is a simplistic description and it is only a theory.
The article also touches on why progesterone treatment might not work for some women.