When Michael reached there, he was shocked to see his wife in racer's clothes. From what he knows about Sara, she only races when she is in a really bad mood and if someone tries to stop her from doing it then all her anger will burst up on that person.
Seeing her in that mood, he starts to regret the decision he made earlier. If he had given her the ring instead of saying that he was buying it for Tina, this would have never happened. Even if he tries to explain to her now, she was not going to listen to him no matter what and will continue her race.
