There was only one word– jealousy. The conversation she had with Margot back at the club provoked feelings she thought was buried deep. The opportunity she lost with the confession of Lucas' intentions, she felt that Margot was simply too selfish. Yes, she did advised her to have sex with Bree, but that was only because she would be telling Andrea about the issue, about Bree's new fling and potential new love interest so that Andrea could scheme a way to get Margot out of their lives.
The minute she advised Margot to do such, Margot actually took her advice, she did not wait for a day or two or even a week and in the course, dropping subtle hints of her plan, going to dinner and such but instead, she invited Bree into her house instantly. And also in front of Sophia, the same Sophia who told her of her feelings for Bree. Sophia was angry at Margot for being such a bitch, for Bree for being promiscuous generally and mostly for herself for being angry.
