Pharamones. Females secrete a certen amount of pegesterone into their urine which males find attractive. I hate when I go to a show and one of my boys finds somewhere a female in heat has been, beaucase they start screaming and ripping at the grass.
Sometimes the females also have bladder infections. The infection maeks the urine more potent and 'sweet' to that special place in the males mind, and they have even more interst in it.
If it is just started, and she is altered, have a urine check done. If she is not spayed, she is pumping out the harmones and he is enjoying it, or she still may have a bladder infection.