Why did Jacobs finally decide to escape in Incidents in the Life of a Slave Girl?
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.
Harriet Jacob's escape from enslavement and relentless quest for manumission were motivated by a desire to arrange for the freedom of her two young children. In her early 20s, Jacobs gave birth to Ellen and Benny. The children were the biracial offspring of Jacobs and a white plantation owner called Mr. Sands.
Before her final escape, Jacobs spends several years hiding out in various crawlspaces and secret chambers of nearby plantations and farms. She depends on the kindness of relatives to help her keep contact with her children. She eventually flees the south on a boat and gains employment in the north. Fearing a forced return to the state from which she fled, Jacobs keeps her identity as a fugitive closely hidden.
Jacobs' brother, who is occasionally assigned whaling trips up north, is able to stay in contact with her through infrequent secret meetings. He informs her that her daughter Ellen had been bought by Mr. Sands. He tells her about the abuse and mistreatment Ellen suffers on the plantation.
Knowledge of her children's continued suffering compels Jacobs to seek freedom. She takes the risk and decides to expose her fugitive status in order to gain help from Northern abolitionists. Jacob's primary motivation for freedom was to secure freedom for her children; she could not accept a future for them in which they would be forced to continue, enslaved and imprisoned on southern plantations for the rest of their lives.