Do we know what causes endometrial cancer?

Many factors. The risk of endometrial cancer is increased by obesity, diabetes, and persistently high estrogen levels, such as may occur with a condition like polycystic ovaries. Genetics also plays a role, especially with lynch syndrome which increases colon, endometrial, and ovarian cancers. Use of oral contraceptives or the Mirena (levonorgestrel) iud can decrease the risk of endometrial cancer.
Estrogen +more. Endometrial cancer is the most common gynecologic cancer. Risk factors for developing endometrial cancer all include over-exposure to estrogen. This can occur with hormone replacement therapy after menopause, starting menstruation at an early age, going through menopause at a late age, never having children, and obesity.
Unopposed estrogen. The majority of endometrial cancers occur in post-menopausal women who are overweight. After menopause the endometrium will continue to grow if stimulated by estrogen (adipose produces estrogen), without ovulation no menstrual period occurs to shed that endometrium. The constantly stimulated endometrium can eventually become a cancer.
We do not know. While medical experts have not determined the cause of uterine cancer (endometrial cancer), it seems clear that an association with duration of estrogen exposure is the major risk factor. This puts women who are obese, had early puberty and no pregnancies, a history of unopposed estrogen (as in hormone replacement with only estrogen) and post breast cancer tamoxifen use at more risk.