Harriet Jacobs, known as Linda Brent in the narrative, finally decides to escape when she finds out that her children are coming to the Flint plantation only to be "broke in." The answer to your question can be found in chapter 16, "Scenes at the Plantation," within the following quote:
My suspicions were correct. My children were to be brought to the plantation to be “broke in.” To this day I feel grateful to the gentleman who gave me this timely information. It nerved me to immediate action.
This knowledge is preceded by suspicion that Dr. Flint, especially due to Linda Brent's refusal of his sexual advances, blames his father for not breaking "her in long ago." Further, Aunt Martha begins to talk about how important it is to "stand by your own children, and suffer with them till death." Both Dr. Flint and Aunt Martha seem to think that Linda's simple proximity to her children will keep her on the plantation and keep her in check. They are wrong. Linda's will to take the "immediate action" of escape ends the chapter.
After telling her friend, Sally, about her escape, Linda then accepts her friend Betty's offer to hide above the stairs. This way, Linda can keep tabs on her children while still escaping slavery. Dr. Flint is furious. He takes out a runaway slave advertisement for Linda and then jails her brother and her children for two months. Unable to find Linda, Dr. Flint finally sells Linda's children to their own father: Mr. Sands. Eventually, Linda switches her hiding place from Betty's house to the tiny crawlspace in her grandmother's attic.