The cows could be pregnant. In late pregnancy the placenta produces large amounts of estrogen to prepare the mammary system for lactation. Estrogen is also the hormone that causes the cow to come into heat, or estrus, and stand. The placental estrogens may be what is causing the riding.
It is wise to have the cows pregnancy verified. If not pregnant, they need to be removed from the calving pastures. Consider culling these cows; their reproductive failure has been costly.
