Ramban: When the wife of Potifar saw that he had left his cloak with her (Seforno, Ha'amek Davar - when she saw that he fled), she was afraid that he would reveal her behavior towards him to the members of her household or to her husband, so she preempted him by lying that he had taken off his cloak in order to be intimate with her, and that it was when she began to scream that he fled.