Seeing that the cards were in his favor, Christian launched his play—immediately closing the distance with Vivian (kissing, touching), aggressively attacking Hutch by questioning his competence and humiliating him with a school-like task. When Vivian saw that her husband was completely failing to handle the situation, she decided to take matters into her own hands and see what Christian had to offer. She might have even taken the initiative herself in the garden—we don’t yet know who started it. Both scenarios are possible, though I lean more toward "You want Hutch to keep his job? Then take off your panties" rather than "I'll sleep with you if you let Hutch stay."