And, to give a much more thoughtful and rigorously looked into answer to your O.P., it seems that while the site I gave you gives a good explanation of what CAUSES the sloughing off, the exact mechanism seems to me to sound almost like Losing a scab. Once the connective cellular layers are rendered useless, the body literally lets go of that material.
In the case of the scab, it falls away cleanly since it’s dry and crusty by that time. In the case of the uterine lining, it’s a bit slower and, shall we say, more cumbersome. I guess due to the slight variations in the death of individual cells, instead of having the blood-rich tissue lining the uterus be expelled all at the same time ( a la the Placental Delivery post-partum ), it happens in shreds and bits over a few days, typically. The endocrine system may trigger it’s release of the tissues, but they all slough off at slightly different moments. ( If I had a method of predicting exactly WHEN most of it would slough off, I’d be a wealthy man and the Prince of Periods )
Also, now that I read that last graph, I realize that the Placenta is not rooted along the entire surface of itself to the uterine wall, but rather is linked in only one spot, the Umbilicus. It makes for an easier departure.