Doctors don't know what causes endometrial cancer. What's known is that something occurs to create changes (mutations) in the DNA of cells in the endometrium — the lining of the uterus.
The mutation turns normal, healthy cells into abnormal cells. Healthy cells grow and multiply at a set rate, eventually dying at a set time. Abnormal cells grow and multiply out of control, and they don't die at a set time. The accumulating abnormal cells form a mass (tumor). Cancer cells invade nearby tissues and can separate from an initial tumor to spread elsewhere in the body (metastasize).