A 41-year-old woman undergoes laparoscopic hysterectomy for symptomatic uterine fibroids. Two days later she develops right flank pain and fever. CT urogram shows contrast extravasation in the pelvis with mild right hydronephrosis, concerning for iatrogenic ureteral injury during the procedure. Which anatomic relationship most directly explains why the ureter is at risk during ligation of uterine vessels in this operation?