What event led to stanleys quest to bring blanche down?

Stanley's quest to bring Blanche down is primarily triggered by her arrival at their home and the disruptive impact she has on his life and marriage to Stella. He feels threatened by her pretentiousness and her attempts to impose her values on their household. The tipping point comes when he uncovers her troubled past, which he uses to expose her vulnerabilities and assert his dominance in the household. This conflict highlights the clash between their different social backgrounds and values.