What prompts Linda to make the decision to escape in Harriet Jacobs?

The catalyst for Linda's decision to escape is Dr. Flint's wicked plan to use her children, Benny and Ellen, as field hands on his plantation.
The children's father is a white man, Mr. Sands. Linda embarked on an affair with him to avoid the unwelcome overtures of Flint. Linda thought that the worst that Flint could do on finding out about the affair would be to sell her to Sands in disgust.
Unfortunately, it doesn't turn out like that. Instead, Flint gets mad and, in a fit of rage, decides that Linda will be broken in on his plantation. If this weren't bad enough, he then determines that Benny and Ellen will suffer the same fate. Linda realizes that there's nothing for it: if she's to protect her children, she must escape. She can't do this with the children in tow; that would be impossible. So instead, she hits on the idea of hiding in a crawlspace in her grandmother's attic. Once Flint finds out that Linda's gone, he'll sell Benny and Ellen to a slave trader representing Sands, who will then free the children at a later date.